Dynamin-related proteins and Pex11 proteins inperoxisome division and proliferation Sven Thoms and Ralf Erdmann Ruhr-University-Bochum, Medical Faculty, Institute of Physiological Chemis
Trang 1Dynamin-related proteins and Pex11 proteins in
peroxisome division and proliferation
Sven Thoms and Ralf Erdmann
Ruhr-University-Bochum, Medical Faculty, Institute of Physiological Chemistry, Bochum, Germany
Introduction
Peroxisomes (or microbodies) are single-membrane
bound organelles comprising plant glyoxisomes,
kineto-plastid glycosomes, Woronin bodies and peroxisomes
in the narrow sense Peroxisomes are very diverse in
their metabolic functions Depending on species, cell
type, and environmental conditions, peroxisomes may
perform different metabolic activities, including fatty
acid a- and b-oxidation, alcohol oxidation, ether-lipid
biosynthesis, glycolysis, and glycerol metabolism [1] In
contrast to their metabolic heterogeneity, the biogenesis
of peroxisomes seems to follow a common pathway,
relying on conserved proteins, the so-called peroxins
Most peroxins are involved in matrix protein import
or in formation of the peroxisomal membrane [2] A
surprisingly large number of peroxins, however, is
required for the proliferation and inheritance of these organelles
The relevance of peroxisomes for human health is underscored by the existence of peroxisomal biogene-sis disorders (PBDs) [3,4] These diseases are charac-terized by defects in peroxisome protein import, which leads to an impairment of all peroxisomal functions, with the accumulation of a- and b-oxida-tion substrates (such as very long chain fatty acids
or phytanic acid) and a reduction in plasmalogen levels PBDs are associated with a number of more pleiotropic abnormalities, such as hypotonia, develop-mental delay, defects in neuronal migration and apoptosis, and hepatic and renal problems At the cellular level, mitochondria can also be affected
in PBDs, probably because of their metabolic inter-relation with peroxisomes [5–7]
Keywords
dynamin-related protein; dynamin;
endoplasmic reticulum; GTPase; organelle
division; peroxisome proliferator-activated
receptor; peroxisome; PEX11; VPS1; yeast
Correspondence
R Erdmann, Systems Biochemistry,
Institute of Physiological Chemistry,
Ruhr-University-Bochum, 44780 Bochum,
Germany
Fax: +49 234321 4266
Tel: +49 234322 4943
E-mail: ralf.erdmann@rub.de
(Received 28 July 2005, accepted 26 August
2005)
doi:10.1111/j.1742-4658.2005.04939.x
The abundance and size of cellular organelles vary depending on the cell type and metabolic needs Peroxisomes constitute a class of cellular organ-elles renowned for their ability to adapt to cellular and environmental conditions Together with transcriptional regulators, two groups of per-oxisomal proteins have a pronounced influence on peroxisome size and abundance Pex11-type peroxisome proliferators are involved in the proli-feration of peroxisomes, defined here as an increase in size and⁄ or number
of peroxisomes Dynamin-related proteins have recently been suggested to
be required for the scission of peroxisomal membranes This review surveys the function of Pex11-type peroxisome proliferators and dynamin-related proteins in peroxisomal proliferation and division
Abbreviations
DRP, dynamin-related protein; GED, GTPase effector domain; PBD, peroxisomal biogenesis disorder; PCD, programmed cell death; PPAR, peroxisome proliferator-activated receptor alpha; PPP, Pex11-type peroxisome proliferators; PPRE, peroxisome proliferator-responsive elements; PRD, proline- and arginine-rich domain.
Trang 2The abundance of peroxisomes in a cell is
regula-ted by a number of as yet incompletely understood
processes These can – at least conceptually – be
divi-ded into (a) peroxisome proliferation by division, (b)
peroxisome de novo biogenesis, (c) peroxisome
inherit-ance, and (d) peroxisome degradation by pexophagy,
an autophagy-related process Our knowledge about
the relative contributions of these processes to
main-tain or establish a cermain-tain number of peroxisomes in
a cell, is rather limited However, at least two classes
of proteins are involved in controlling peroxisome
number and division This review offers an overview
of these two classes, namely dynamin-related proteins
(DRPs) [8], and Pex11-type peroxisome proliferators
(PPPs) Proliferation is understood here as a process
that leads to an increase in size and⁄ or number of
peroxisomes
Peroxisome proliferation at large
The idea of peroxisome biogenesis by ‘growth and
division’ was put forward in a very influential review
20 years ago [9] Based on the post-translational
import of matrix proteins and one major membrane
protein [10,11], it has become a largely accepted
dogma that membrane proteins, as well as matrix
pro-teins, are imported post-translationally from the
cyto-sol In the light of recent research, however, a
substantial contribution from the endoplasmic
reticu-lum seems likely [12–16]
In yeast, fatty acids cause the proliferation of
per-oxisomes [17] and the transcriptional up-regulation of
peroxisomal b-oxidation enzymes This response is
mediated by the oleate response element, together with
the transcription factor complex, Pip2–Oaf1 [18–20],
and the transcription factor, Adr1 [21,22] Adr1
regu-lates expression of the peroxisome-specific acyl-CoA
oxidase FOX1⁄ POX1 as well as of PEX11 [23]
Early work on peroxisome division in Candida
boidiniihas shown that small peroxisomes carrying an
incomplete set of matrix proteins divide and mature by
protein import only after a large number of immature
peroxisomes have been formed [24] This work was
extended by a comparative study using different
growth conditions to induce peroxisomes [25] It was
found that certain peroxisome-inducing conditions,
such as d-alanine, methanol or oleate, up-regulate
per-oxisome-resident enzymes in a specific manner, rather
than causing a general increase in peroxisome number
These findings underscore the variability and versatility
of these organelles
Five different immature peroxisome populations
have been identified in the yeast Yarrowia lipolytica,
which are described to mature by movement through
an ordered pathway [26] In the course of peroxisome maturation, acyl-CoA oxidase moves in a heteropenta-meric complex from the matrix to the inner membrane
of the peroxisome The membrane-bound pool of acyl-CoA oxidase interacts with Pex16, which is also mem-brane bound inside the peroxisome The substrate– Pex16 interaction inhibits the negative influence of Pex16 on peroxisome division and thereby allows per-oxisome division [27]
‘Growth and division’ do not follow the same course
in all species In Y lipolytica, and similarly in Hansen-ula polymorpha, peroxisomal vesicles do not divide before they have matured after the import of matrix proteins [28,29] In contrast, in C boidinii, immature peroxisomes that have only acquired part of their matrix protein content seem prone to divide [30] In human cells, however, both mature and immature per-oxisomes have the capability to divide [31] Whether these differences truly reflect species differences, or if they are a result of different methods, remains to be evaluated
In mammalian cells, peroxisome proliferator-activa-ted receptor alpha (PPARa) is critical for peroxisome induction [32] PPARa belongs to the superfamily of ligand-activated nuclear transcription factors [33–36] The ligands of these receptors are lipids, lipophilic substances, together with synthetic hypolipidaemic drugs, or peroxisome proliferators PPARs bind to peroxisome proliferator-responsive elements (PPREs)
in a heterodimer with retinoid X receptor PPARa is expressed in adipose tissue and liver Its target gene products are involved in lipid catabolism such as fatty acid uptake, storage and oxidation (in peroxi-somes and mitochondria), and in lipoprotein assem-bly and transport Two other PPAR subtypes have been described: PPARb (¼ PPARd) and PPARc PPARb is ubiquitously expressed, and PPARc is expressed mainly in adipose tissue, but also in colon, the immune system, and in the retina PPARc con-trols the differentiation of adipose tissue and fatty acid storage and mobilization In spite of their name, PPARb and PPARc have not been associated with peroxisome proliferation PPARs are involved in dis-eases such as diabetes, obesity, atherosclerosis, and cancer, which explains the high interest in pharmaco-logical control of these proteins A clear-cut evalua-tion of PPAR effects, however, is hampered by species differences between rodents and humans, which might, in part, be explained by different expression levels [37] resulting from differences in the PPREs [38], leading to nonconserved responses to peroxisome proliferators
Trang 3Pex11 proteins in peroxisome
proliferation
Pex11 was the first protein identified as being
involved in peroxisome proliferation or division in
yeast [39,40] Loss of PEX11 leads to reduced
per-oxisome abundance with giant peroxisomes [39]
Similarly, depletion of Trypanosoma brucei PEX11
(TbPex11) reduces glycosome number and size [41]
Conversely, overexpression of PEX11 promotes
per-oxisome elongation and proliferation in yeast [40],
and TbPex11 overexpression causes elongation and
clustering of glycosomes [41]
Pex11-mediated peroxisome division is described as
a process consisting of up to four partially overlapping
steps [42], namely (a) the insertion of Pex11 into the
membrane, (b) the elongation of peroxisomes, (c) the
segregation of Pex11 and the formation of
Pex11-enriched patches, and (d) the division of peroxisomes
(Fig 1)
In all organisms studied to date, microbody
abun-dance can be increased by the expression of extra
copies of PEX11 Recently, this was confirmed for
Penicillium chrysogenum, where PEX11 overexpression
likewise leads to the proliferation of microbodies and
an increase in penicillin production, which is not
accompanied by a significant increase in penicillin
biosynthesis enzymes [43] PEX11-induced penicillin
overproduction in P chrysogenum could be explained
by increased metabolite transport through the
micro-body membrane and might prove commercially
rele-vant
Pex11 function has mostly been analysed in
Sac-charomyces cerevisiae, trypanosoma, and mammals
Diverse as the peroxisome functions are in these
organisms, a requirement of the three Pex11 isoforms
seems to be a common factor
Three Pex11 isoforms in mammals
In mammals, three isoforms of Pex11 – Pex11a, Pex11b, and Pex11c – have been identified [42,44,45] All three isoforms are described as membrane proteins with two transmembrane domains and both termini exposed to the cytosol
PEX11a is inducible by inducers of peroxisome pro-liferation Its expression is highest in liver, kidney, heart, and testis [42,45–47] A PEX11a knockout mouse is morphologically indistinguishable from a wild-type mouse, with no obvious effect on peroxisome number or metabolism [47], suggesting that its loss can
be largely compensated by other Pex11 isoforms The induction of peroxisome proliferation through PPARa
by ciprofibrate does not require PEX11a, but leads to the clustering of mitochondria around lipid droplets and abnormally straight mitochondrial cristae [47] In contrast, the nonclassical peroxisome proliferator, phenylbutyrate, works independently of PPARa, but
is PEX11-dependent [47] Phenylbutyrate also induces the adrenoleucodystophy-related gene (ALDP) [48] The second isoform of Pex11, Pex11b, is not indu-cible by peroxisome proliferators It is constitutively expressed in most tissues [44] Overexpression of PEX11b induces peroxisome proliferation to a greater extent than overexpression of PEX11a [42] The knockout of PEX11b in mice leads to neonatal lethality with a number of defects reminiscent of Zellweger, including developmental delay, hypotonia, neuronal migration defects, and neuronal apoptosis [49] These mice are, however, only mildly affected
in peroxisome protein import and metabolism (reduced ether lipid biosynthesis) [49] This prompts the idea that some of the pathological features of Zellweger are not caused by gross metabolic distur-bances but rather by subtle effects on signalling pathways involving peroxisomal substrates or prod-ucts In cases where only a limited number of metabolites would have to be normalized, this could raise hope for therapeutic intervention in peroxi-somal diseases
Knockout mice with deletion of both PEX11a and PEX11b still contain peroxisomes and are only mildly affected in peroxisomal metabolic activity [49] These mice also die early after birth with severe neurological defects [49] In summary, PEX11a seems to be respon-sible for peroxisome proliferation in response to exter-nal stimuli, whereas PEX11b is required for constitutive peroxisome biogenesis
The third isoform, Pex11c, is constitutively expressed in liver [50] and might have a redundant function with Pex11b, although it is with 22% amino
Fig 1 Model of peroxisome proliferation and division (1)
Elonga-tion (2) SegregaElonga-tion (3) ConstricElonga-tion (4) Fission ⁄ division For
details see the text.
Trang 4acid identity less similar to Pex11b than Pex11a is to
Pex11b (40% amino acid identity)
New members of the PEX11 family in yeast
Recently, proteins with a weak similarity to Pex11
have been identified in S cerevisiae [51–53] The new
Pex11 proteins, Pex25 and Pex27, are more similar to
each other (18% identity) than to Pex11 ( 9%
iden-tity) Pex25 (45 kDa) and Pex27 (44 kDa) have a
higher molecular mass than Pex11 (27 kDa); in fact,
they appear to have an N-terminal extension when
compared with Pex11 All three proteins localize to
peroxisomes Pex25 behaves as a peripheral membrane
protein [51]
The knockout of PEX25 has a stronger growth
defect on oleic acid than the deletion of PEX27 The
double knockout of PEX25 and PEX27 has about the
same growth defect on oleic acid as the PEX25 single
knockout [51,52] Growth of this double deletion can
be restored by low copy expression of PEX25 or high
copy expression of PEX27 [51] The triple deletion of
all three PPPs is unable to grow on oleic acid [51],
indicating that at least one of the Pex11 proteins is
required for peroxisome biogenesis The growth defect
of the triple mutant can be alleviated by the
over-expression of PEX25, but not by the overover-expression of
PEX27 or PEX11 [51] The triple deletion shows a
matrix protein import defect, even under conditions
where peroxisome proliferation is not induced by oleic
acid [51] In the triple mutant, thiolase is expressed at
normal levels, indicating that Pex11 family members
are not involved in fatty acid signalling
Single and double deletions of members of the PPPs
contain enlarged peroxisomes [51–53], underscoring the
idea that Pex11 proteins are involved in peroxisome
proliferation Conversely, the overexpression of each
of the family members causes peroxisome proliferation
or enlargement [51,52] The overexpression of PEX25
also causes kamellae around the nucleus [51] PEX25 is
induced by oleic acid [53,54] through an unusual oleate
response element in its promotor [54], whereas PEX27
is not induced at all on oleic acid [51,52] Thus, in
oleic acid-induced cells, the Pex11 expression level is
highest and the Pex27 expression level lowest All
Pex11 proteins interact with themselves [51,52,55]
They are likely to form oligomers or
homo-dimers Additionally, Pex25 and Pex27 interact with
each other [51,52]
In trypanosoma, there are also two additional Pex11
isoforms, GIM5A and GIM5B These two proteins are
nearly identical in sequence and show weak similarity
to Pex11 Both are 26 kDa, have two putative
transmembrane domains, and assemble into hetero-di-mers [56] A GIM5 reduction leads to a lower phos-phatidylcholine⁄ phosphatidylethanolamine ratio and a decrease in ether lipids [57], which could increase membrane fluidity Trypanosomes with reduced GIM5 levels have enlarged glycosomes, which are more fra-gile than wild-type glycosomes [57]
Thus, it turns out that mammals, S cerevisiae and trypanosomes have three Pex11 homologues each Whether they represent an early or a late diversifica-tion of an ancestral Pex11 funcdiversifica-tion could not be deter-mined because of their low sequence similarity
Pex11 and perilipin
In mouse, PEX11a and the lipid body protein perilipin are regulated from a single PPRE that is situated between the two genes [58] As a consequence of this gene arrangement, PEX11a, which is expressed mainly
in the liver, and perilipin, whose expression is limited
to adipose tissue, can be competitively regulated by PPARa and PPARc, respectively This is not only a noticeable example of gene clustering in mammals [59],
it also indicates that peroxisome proliferation can be induced by switching from PPARc to PPARa Fur-thermore, the common regulation of Pex11 and peri-lipin indicates metabolic association of peroxisomes with lipid storage function [60]
New proteins affecting peroxisome number Pex28 and Pex29 are two recently identified proteins with a weak similarity to Pex24 from Y lipolytica Pex24 is an oleic acid-inducible peroxisomal integral membrane protein that is required for growth on oleic acid [61] Mutants of PEX24 have no apparent peroxi-somes, they mislocalize peroxisomal matrix and mem-brane proteins, yet contain vesicular structures with some peroxisomal proteins [61] Pex28 and Pex29 from
S cerevisiae are also peroxisomal membrane proteins [62] They are, however, not inducible by oleic acid Double or single deletions of the two proteins show an increased number of small and clustered peroxisomes Pex23 from Y lipolytica is an oleic acid-inducible membrane protein [63] Three proteins from bakers yeast, which have been termed Pex30, Pex31, and Pex32, show sequence similarity to Pex23 and have also been localized to the peroxisomal membrane [64] Pex30 and Pex32 are induced by oleic acid These new peroxins are partially redundant and partially interact with each other Deletions of these latest additions to the PEX list show an increase in peroxisome numbers, enlarged or clustered peroxisomes, so that they have
Trang 5been described as regulators of peroxisome size and
number [64] Based on an epistasis analysis, Pex30–32
are placed downstream of Pex28 and Pex29 [64]
Models for Pex11 function
The eight peroxins – Pex11, Pex25, Pex27, Pex28,
Pex29, Pex30, Pex31, and Pex32 – have a more or less
pronounced effect on peroxisome size and number To
date it is unclear how this effect is exerted Different
explanations are possible, as follows:
(a) Some of these proteins might be directly involved
in fatty acid metabolism [65] Yeast mutants lacking
PEX11 exhibit a defect in the b-oxidation of
medium-chain fatty acids [65] On this basis, it was suggested
that Pex11 plays a primary role in medium-chain fatty
acid metabolism and promotes peroxisome division
only indirectly [65] In addition, there is evidence that
Pex11 can promote peroxisome proliferation in the
absence of metabolism [66]
(b) The peroxins might be metabolite transporters or
porins [57] This would, however, require a rather
broad substrate specificity of these proteins, with fatty
acid and glycolytic substrates being transported in
clas-sical peroxisomes and glycosomes, respectively
(c) They might be structural components of the
per-oxisomal membrane For PPPs such an explanation is
likely, yet nonexclusive with other explanations They
are by far the most abundant proteins of the
peroxi-somal membrane (shown in yeast and trypanosomes)
Thus, they might directly and specifically shape the
peroxisomal membrane Overexpression of other
per-oxisomal membrane proteins has been reported not to
induce peroxisome proliferation [66]
(d) They might recruit other proteins to the
mem-brane The recognition of Pex25 as a receptor for the
GTPase Rho1 [67] could be a first step of research into
this direction
In summary, there are some models on how PPPs
(together with Pex30 to Pex32) might affect
peroxi-some number These models are nonexclusive with
each other, and the mechanism of action will not be
the same for all PPPs In the light of the different roles
that have been suggested for Pex11, it is possible that
PPPs are multifunctional enzymes Recently, another
class of proteins has come into focus These are
sug-gested to affect peroxisome division in a more direct
way
DRPs in peroxisome division
Before addressing the role of DRPs in peroxisome
division, we will briefly introduce (a) conventional
dynamins, (b) the structural and physicochemical prop-erties of DRPs and (c) DRPs engaged in the division
of endosymbiotic organelles
Dynamins are involved in endocytosis and intracellular trafficking
Dynamins are GTPases involved in intracellular fis-sion processes [68–70] Five domains have been identi-fied in conventional dynamins: a highly conserved N-terminal GTPase domain, a less conserved ‘middle domain’, and a pleckstrin homology domain that mediates interactions with phosphatidylinositol-phos-phates (Fig 2) The C terminus comprises the GTPase effector domain (GED), which activates GTPase activity and mediates self-assembly, and a proline and arginine-rich domain (PRD) that mediates interactions with SH3 domains of effector proteins of the actin cytoskeleton
Dynamins are required in phagocytosis and in caveo-lae- and clathrin-dependent endocytosis [71] Of the three conventional mammalian dynamins, Dynamin1 is neuron-specific, Dynamin2 is expressed in all tissues and Dynamin3 is found in brain, lung, heart, testis and blood cells
The role of dynamin in clathrin-mediated endocytosis emerged from the study of the temperature-sensitive mutant shibire in Drosophila melanogaster [72] Shibire shows a paralytic phenotype that is probably caused by
a defect in the reuptake of synaptic vesicles at the presy-naptic membrane and subsequent sypresy-naptic vesicle deple-tion at the neuromuscular juncdeple-tion [73] Electron micrographs of shibire nerve termini show the formation
of clathrin-coated buds unable to sever from the mem-brane Dynamin localizes to the necks of these buds [73– 75] Recently, a mutation in the PH domain of DNM2 has been identified as the cause of one form of Charcot-Marie-Tooth disease, a neuromuscular degenerative dis-order [76], thereby providing the first link between a classical dynamin and an inheritable human disease
Dynamin biochemistry and structure
In vitro, dynamin assembles into rings upon dilution into buffers of low ionic strength [77] Furthermore,
Fig 2 Domain structure of dynamins and dynamin-related proteins (DRPs) GED, GTPase effector domain; MD, middle domain; PH, pleckstrin homology; PRD, proline- and arginine-rich domain.
Trang 6dynamin can self-assemble into spiral-like structures
around liposomes in vitro, tubulate them and,
depending on the lipid composition, cause them to
vesiculate [78–80] It is a matter of debate whether
dynamins in vivo are involved in both the
constriction and the scission of vesicular membranes
[68]
Dynamin has been described as a force-generating
mechanochemical enzyme using GTPase-dependent
conformational changes to drive fission directly either
by consticting [82] or extending the necks of coated
pits [82,83] These models are referred to as the
‘pin-chase’ and the ‘poppase’ model, respectively [78,84]
Dynamins are characterized by a low affinity for
GTP and GDP, which makes them independent of
a guanidine nucleotide exchange factor The GED
functions as a GTPase-activating protein Upon
homo-oligomerization, the GTPase activity is greatly
stimulated [81], providing support for the pinchase
model
An alternative model is based on experiments with
dynamin GED mutants that have lost
GTPase-activa-ting protein function, yet, when overexpressed in baby
hamster kidney cells, stimulate endocytosis [85,86]
This gave rise to the idea that dynamin, like other
members of the GTPase superfamily [87] works as a
molecular switch by effecting downstream activators of
membrane fission processes [88] In all models on
dyn-amin function, however, dyndyn-amin assembles at the
neck of membrane invaginations that are later to be
fissioned
A large number of cytoskleletal proteins interact
with dynamins in endocytosis, often via the PRD
domain of dynamin [89–94] These include profilin,
Abp1, syndapin, intersectin and cortactin Recently, it
has been shown that the yeast DRP, Vps1, is also
required for normal actin organization and that it
interacts with the actin regulatory protein, Sla1 [95]
Dynamin proteins might link the cytoskeleton to
vesi-cles [90,92] Information on the dynamics of dynamins
in vivo has been obtained by evanescent wave
micro-scopy, which allowed a time-resolved analysis on how
clathrin-coated pits move inwards from the plasma
membrane In this study, a consecutive recruitment of
dynamin and actin was observed [96] Thus, dynamin
may be the precondition for actin assembly Recently,
it has been shown that Dynamin2 functionally
inter-acts with the actin-binding protein cortactin not only
at the cell membrane but also at the Golgi apparatus
[97], so that a model which unites the function of
dyn-amin proteins at various cellular sites and their mode
of interaction with the cytoskeleton now seems within
reach
DRPs DRPs share, with classical dynamins, the N-terminal GTPase domain, a middle domain, and the C-terminal GED (Fig 2) Obvious PRDs or PH domains are not found in DRPs Examples of DRPs are Mx proteins, mammalian DLP1, and the yeast proteins Dnm1, Mgm1 and Vps1
Mx proteins are interferon-inducible DRPs [98] Like dynamin, they self-assemble and bind and tubu-late lipids, a function that might not be required for their antiviral activity Mx proteins are found in association with the endoplasmic reticulum Mx pro-teins are able to shield cells from infections with RNA viruses It is hypothesized that they do so by binding
to the viral nucleocapsid and either promoting its degradation or preventing its nuclear entry
DLP1 is a mammalian DRP required for the main-tenance of mitochondrial morphology and division [99,100] It is localized to mitochondria, but not exclu-sively [101,102] DLP1 oligomerizes and has mechano-chemical properties similar to dynamin [103,104] The yeast genome does not encode a conventional dynamin
Of the three DRPs, Dnm1 and Mgm1 are involved in mitochondrial fission and fusion, and Vps1 is required for peroxisome morphology and for protein trafficking
to the vacuole (vacuolar protein sorting) [105–107] Vps1 also participates in clathrin-dependent trafficking from the Golgi via a prevacuolar compartment to the plasma membrane This pathway leads to the synthesis
of high density secretory vesicles, and is also dependent
on the SNARE Pep12 [108]
DRPs in the division of endosymbiotic organelles Endosymbiotic organelles rely, for their division, on a combined machinery, which is derived partly from the host and partly from the endosymbiont [109–111] Symbiont-derived proteins include FtsZ proteins, which are GTPases FtsZ proteins might be ancient relatives of dynamins This would, however, not be supported by structural data: FtsZ is a structural homologue of tubulin [112], whereas dynamin has a different fold belonging to the GTPase superfamily [113] In mitochondria of nearly all species, the dependence on these FtsZ-type symbiont-derived cyto-solic factors has been lost
Two of the three yeast DRPs, Dnm1 and Mgm1, are involved in mitochondria morphology and inherit-ance Mgm1 is required for mitochondrial inner mem-brane fusion [114,115] Defects in its human homolgue, OPA1, are associated with optical atrophy type I [116,117] Mgm1 is present in two essential isoforms in
Trang 7the intermembrane space of mitochondria The shorter
isoform is derived from the longer by processing
[118,119] by the rhomboid protease Pcp1 [120] The
two GTPases, Fzo1 and Mgm1, are linked on the
outer mitochondrial membrane by Ugo1 [121]
Dnm1 is involved in mitochondrial fission [122,123]
and can be regarded as an counterplayer of Fzo1,
which might also be a distant relative of dynamin
Loss of Dnm1 leads to the formation of a
mitochond-rial net throughout the yeast cell, whereas loss of Fzo
leads to mitochondrial fragmentation as a result of
uncompensated mitochondrial fission [124–126] At the
division site of mitochondria, Dnm1 forms a complex
with the WD protein, Mdv1 [127,128], and the TPR
protein, Fis1 [129,130] Recently it has become possible
to study yeast mitochondrial fusion in vivo [131] The
double membrane of mitochondria necessitates a
com-plex node of division, with many GTPases working
together Peroxisomes might offer a simpler system for
studying the action of DRPs
ARC5 is a DRP required for chloroplast division
[132] It is localized to a ring at the chloroplast
divi-sion site and might represent the 5 nm
outer-plastid-dividing ring Mutants of ARC5 have a reduced
number of enlarged, dumbbell-shaped chloroplasts
[133] Interestingly, they are still constricted, but
can-not divide [134]
Similarly to DLP1, DRP-1, the DLP1 homologue of
Caenorhabditis elegansis involved in the scission of the
mitochondrial outer membrane [135] DRP-1 is further
required to induce mitochondrial fragmentation and
programmed cell death (PCD) [136] The
overexpres-sion of DRP-1 can induce PCD, indicating an
evolu-tionary conservation of mitochondrial involvement in
PCD The yeast homologue of DPR-1, Dnm1, might
also be involved in PCD [137]
The parasitic eukaryote T brucei contains only a
single mitochondrion, which undergoes extensive
remodelling during the life cycle of the trypanosome
The genome of T brucei, however, like those of
Leish-mania major and T vivax, encodes only a single
dyn-amin, which is required for mitochondrial fission and
not for endocytosis [138] This points to an original
role of dynamins in organelle division, rather than
endocytosis At the same time it suggests that all
dyn-amin-dependent organelles of these eukaryotes would
have to rely on the same dynamin for division
DRPs in the division of peroxisomes
The mammalian DRP, DLP1, partially localizes to
peroxisomes and is involved in peroxisome fission Its
peroxisomal localization is more readily visible when
peroxisome proliferation is induced by the overexpres-sion of PEX11b [139,140] DLP1 is also found in immunopurified peroxisomes [140] and is enriched in the peroxisomal fraction when peroxisome prolifer-ation is stimulated by bezafibrate [139] Fis1, a DLP1-interacting protein, known to function in mito-chondrial fission, was also found to play a role in per-oxisomal fission, and might act as an adaptor for DLP1 [140a]
Overexpression of the dominant negative K38A GTPase domain mutation of DLP1 (which inhibits GTP hydrolysis, but does not affect GTP binding) leads to pronounced tubulation of peroxisomes when PEX11b is co-expressed [139] Inhibition of DLP1 by expression of the dominant negative form of DLP1 also affects the morphology of mitochondria, but did not change the distribution of peroxisomes in the cell [139] For the overexpression of a S39N mutation in the GTPase domain (reduced GTP affinity) in a DLP1 isoform, a reduction in peroxisome number has been reported, whereas overexpression of wild-type DLP1 has no effect on peroxisome abundance [140] To a les-ser extent, peroxisome tubulation was also obles-served when PEX11b was not overexpressed [139]
An RNAi knock-down of DLP1 in COS-7 (green monkey kidney) cells leads to elongated peroxisomes with a segmented appearance [141], whereas an RNAi knock-down of DLP1 in immortalized human fibro-blast cells leads to a reduction in peroxisome abun-dance [140] In the absence of DLP1, peroxisomes are still able to constrict, yet not able to divide, suggesting that the DRP DLP1 is required for division, but not for constriction [141] Concomitant overexpression of PEX11b induces further elongation of peroxisomes and results in what appears to be a peroxisomal net-work [141] However, when DLP1 was reduced by RNAi, overexpression of PEX11b could no longer induce peroxisome proliferation [140]
In summary, DLP1 seems to be involved in the fis-sion of peroxisomes Overexpresfis-sion of PEX11 causes peroxisome division in a multistep process with elonga-tion first, and then division DLP1 is believed to be required for the division step only (Fig 1)
The involvement of a DRP in peroxisome division was first observed in yeast [142] A deletion mutant of VPS1 contains only a few enlarged peroxisomes (Fig 3), which, by electron microscopic analysis, appear as ‘beads on string’, that is, constricted organ-elles before fission [142] A partial co-localization of Vps1 with peroxisomes has been observed [142] DLP1 shows a higher sequence similarity to yeast Dnm1 than
to Vps1; however, Dnm1 does not influence per-oxisome division under normal growth conditions
Trang 8[142,143] Together with Vps1, actin [142] and the type
V myosin, Myo2, are required for peroxisome
inherit-ance [142]
DRP involvement in peroxisome fission is also
found in plants In the Arabidopsis thaliana DRP3A
mutants, peroxisomes are elongated and reduced in
number [144] These mutants also show an aberrant
mitochondrial morphology [144]
Co-operation of PPPs and DRPs?
Interestingly, a double deletion of PEX28 and PEX29
can be complemented by the overexpression of Vps1
or Pex25, indicating a genetic interaction of members
of the two protein families [62] However, loss of the
DRPs DLP-1 or Vps1 has a more stringent effect on
peroxisome number reduction than the loss of Pex11
Conversely, Pex11 overexpression can induce
per-oxisome proliferation, whereas DRP overexpression
does not have such an effect Based on these findings,
it may be speculated that DPRs are part of the
per-oxisome division machinery, whereas Pex11 family
members act earlier by causing membrane elongation
or recruitment of components of the division
machin-ery (Fig 1) However, attempts to demonstrate a
phys-ical interaction between PPPs and DRPs have not yet
been successful [140] Thus, if there is a physical
inter-action between PPPs and DRPs it is probably indirect
or transient
Knowing that DRPs are generally involved in
organ-elle division, and observing that the disruption of
DRP1 or Vps1 leads to peroxisome enlargement, it
may seem an obvious interpretation that these proteins
are required for peroxisome division This
interpret-ation, however, is based on the assumption that
peroxisomes arise by growth and division, rather than
by de novo biogenesis from heterologous intracellular membranes In the light of peroxisome biogenesis in association with the secretory pathway [16], and DRPs being mainly localized to the endoplasmic reticulum and to the Golgi apparatus, it is possible that DRPs are not primarily involved in peroxisome division, but also in the biogenesis from peroxisomes as they eman-ate from their precursors In this scenario, the steps leading to peroxisome formation depicted in Fig 1 would reflect peroxisome biogenesis rather than per-oxisome division
Dynamins have been given a central role in the evo-lution of the eukaryotic cell [109] Likewise, Pex11 pro-teins might share a long evolutionary history [145] Thus, elucidation of the roles of these proteins in per-oxisome function will be of interest to cell biologists and evolutionary biologists alike
Acknowledgements
We thank Michael Schrader and Hartmut Niemann for reading the manuscript This work was supported
by grants from the Deutsche Forschungsgemeinschaft (SFB642 and ER178⁄ 2-4) and by the Fonds der Chem-ischen Industrie
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