Biochemie-Zentrum Heidelberg, Im Neuenheimer Feld 328, 69120 Heidelberg, Germany
* Author for correspondence (e-mail: walter.nickel{at}urz.uni-heidelberg.de )
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Summary |
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Key words: Coatomer, ARF, p24 proteins, Vesicular transport, Coat assembly, Protein secretion, Golgi
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Introduction |
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COPI vesicles have been used widely as a model system to study the
molecular mechanism of transport vesicle biogenesis since they can be
generated from purified Golgi membranes in vitro and isolated in appreciable
amounts (Malhotra et al.,
1989; Rothman,
1994
). Here, we discuss the basic molecular components of COPI
vesicles and their coordinated interplay in coat assembly and disassembly in
the context of results obtained from various in vitro systems that
reconstitute COPI vesicle biogenesis.
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Recruitment of COPI to Golgi membranes |
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Nucleotide exchange on ARF1 has widely been regarded to be the initial step
of COPI coat assembly (Rothman,
1994). However, recent data demonstrate that ARF1-GDP interacts
with the Golgi in a specific manner
(Gommel et al., 2001
) and that
this interaction precedes nucleotide exchange. On the basis of crosslinking
studies as well as ARF1-binding experiments employing native Golgi membranes,
p23, a type I transmembrane protein known to play a key role in COPI coat
assembly (Sohn et al., 1996
;
Bremser et al., 1999
) was
identified as an ARF1-GDP receptor (Gommel
et al., 2001
). p23 belongs to the p24 protein family, members of
which share common structural features such as double lysine or double
arginine residues in their cytoplasmic tails (for a review, see
Emery et al., 1999
). While the
first member was identified in 1991 (Wada
et al., 1991
), the existence of a family of related proteins was
reported in 1995 by Rothman and colleagues
(Stamnes et al., 1995
).
Currently, six family members have been identified in higher eukaryotes,
whereas eight are known in yeast (Emery et
al., 1999
). The identification of p23 as an ARF-GDP receptor
(Gommel et al., 2001
) is
consistent with recent in vivo studies demonstrating energy transfer between
p23-CFP and ARF1-YFP in living cells, an interaction detected only under
conditions that allow ARF-mediated GTP hydrolysis
(Majoul et al., 2001
).
Peptide-mapping studies revealed that the p23-interacting domain is located
within the C-terminal 22 residues of ARF1
(Gommel et al., 2001
).
Since GEF-catalyzed nucleotide exchange on ARF1 can take place in the
presence of liposomes lacking any proteinaceous factors
(Beraud-Dufour et al., 1999),
the ARF1-GDP-p23 interaction cannot be a prerequisite for nucleotide exchange.
However, in the context of a native membrane, p23 might direct ARF1-GDP to
subdomains of the Golgi that are active in COPI vesicle formation and thus to
its GEF. In addition, ARF-mediated GTP hydrolysis is required during early
stages of the budding process to allow efficient uptake by COPI vesicles of
various cargo molecules (Nickel et al.,
1998b
; Malsam et al.,
1999
; Pepperkok et al.,
2000
). Interestingly, ARF-mediated GTP hydrolysis is
differentially affected by members of the p24 protein family and this is
implicated in their sorting into distinct classes of COPI vesicle
(Goldberg, 2000
). Thus,
ARF1-GDP is likely to be produced continuously during the budding process.
Since it is only ARF-GTP that stably interacts with the membrane [through an
exposed myristic acid residue covalently attached to the amphipathic
N-terminus of ARF1 (Goldberg,
1998
], some mechanism must efficiently retain ARF1-GDP in the
budding zone. Since p23 belongs to the core machinery of COPI budding (see
below), the observed interaction between ARF1-GDP and p23 may well have such a
function.
Upon nucleotide exchange, ARF1-GTP dissociates from p23
(Gommel et al., 2001). As a
result, two binding sites for coatomer are generated, on ARF1-GTP
(Zhao et al., 1997
;
Zhao et al., 1999
) and p23
(Sohn et al., 1996
;
Dominguez et al., 1998
). p23,
and p24, another member of the p24 protein family
(Stamnes et al., 1995
), exist
in various oligomeric forms
(Füllekrug et al., 1999
;
Gommel et al., 1999
;
Marzioch et al., 1999
), which
might be important for COPI vesicle formation (see below). Despite the
abundance of p23 and p24 in the Golgi, coatomer binding strictly depends on
the preceding activation and membrane recruitment of ARF1
(Donaldson et al., 1992a
;
Palmer et al., 1993
). In
principle, at least two scenarios would be consistent with this observation.
First, activated and thus membrane-associated ARF1 could simply change the
equilibrium between cytosolic and Golgi-associated coatomer pools on the basis
of its GTP-dependent direct interaction with coatomer
(Zhao et al., 1997
;
Zhao et al., 1999
). In this
case p24 proteins in the Golgi would be constitutively active in binding
coatomer; however, ARF-GTP recruitment would efficiently redistribute coatomer
from the cytosol to the membrane. A second possibility would be that binding
of ARF-GTP to the Golgi initiates a process that converts p23/p24 oligomers
from an inactive state into a state active with regard to coatomer binding
activity. Such a mechanism is supported by the fact that ARF-GDP binds to p23
and, upon nucleotide exchange, dissociates from p23
(Gommel et al., 2001
),
suggesting that multiple cycles of ARF-GDP-p23 complex formation,
nucleotide-exchange-mediated dissociation and GTP-hydrolysis-mediated
regeneration of ARF-GDP might act on the oligomeric status of p23/p24
complexes. A possible structural basis would be an ARF-GTP-dependent
rearrangement of p23/p24 oligomers for example, the conversion of
homooligomers into heterooligomers. Such a process could be driven by cycles
of GDP-for-GTP exchange on ARF and GTP hydrolysis
(Fig. 2, grey box), a process
known to be required for later stages of COPI vesicle biogenesis
(Nickel et al., 1998b
;
Malsam et al., 1999
;
Pepperkok et al., 2000
). It is
of note that both working models are based on a bivalent interaction of
coatomer with the membrane as well as explain why binding to Golgi membranes
of coatomer strictly depends on the preceding activation and membrane binding
of ARF1 (Donaldson et al.,
1992a
; Palmer et al.,
1993
).
|
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COPI polymerization |
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How exactly is coat polymerization triggered and does this process involve
a conformational change in the coat protein itself? These questions have been
addressed by in vitro experiments employing purified coatomer and synthetic
peptides that correspond to the cytoplasmic domains of the coatomer-binding
proteins p23 and p24 (Sohn et al.,
1996; Dominguez et al.,
1998
). As a control peptide, the cytoplasmic domain of the yeast
protein Wbp1 (te Heesen et al.,
1992
) was used in these studies because it is structurally and
functionally related to some p24 proteins in its C-terminal sequence and binds
coatomer. In the case of Wbp1, an ER-resident protein, coatomer binding allows
its retrieval from the Golgi to the ER
(Cosson and Letourneur, 1994
;
Letourneur et al., 1994
) and
thus makes the protein cargo rather than machinery. The various peptides were
designated p23-CT, p24-CT and Wbp1-CT (with CT for cytoplasmic tail) in order
to reflect their origin. Low concentrations of preformed homodimers of p23-CT
and p24-CT peptides promote aggregation of soluble coatomer
(Reinhard et al., 1999
). By
contrast, dimeric Wbp1-CT has no such effect, which is consistent with a
non-machinery nature of Wbp1. Dimeric p23-CT and p24-CT spontaneously form
stable tetramers with a defined secondary structure as determined by mass
spectrometry and NMR (Fligge et al.,
2000
; Weidler et al.,
2000
). This observation is consistent with the 4:1 stoichiometry
of p23-CT to coatomer determined in aggregates
(Reinhard et al., 1999
); the
same ratio is also found in native COPI-coated vesicles
(Sohn et al., 1996
).
Interestingly, limited proteolysis revealed that coatomer aggregation is
accompanied by a conformational change in
-COP, the coatomer subunit
that directly contacts p23 (Harter and
Wieland, 1998
; Reinhard et
al., 1999
). This study has established two conformations of
coatomer: that of the soluble coatomer; and that of coatomer in COPI-coated
vesicles or aggregates. Coatomer aggregation in the presence of tetrameric
p23-CT thus appears to be related to COPI coat polymerization on native
membranes, which suggests that the interaction of p24 proteins with coatomer
is a critical trigger of COPI coat assembly.
Interestingly, preformed p23-CTp24-CT heterodimers appear to
stimulate coatomer aggregation even more efficiently than do p23- or p24
homodimers (C. Reinhard and F.T.W., unpublished). This observation might
indicate that a specific configuration of p23/p24 oligomers is required for
the initiation of COPI coat assembly (Fig.
2). As discussed above, binding of coatomer to Golgi membranes
requires prior activation and membrane binding of ARF1. Therefore, it is
conceivable that rearrangements of p23/p24 oligomers occur during the overall
process of COPI coat assembly, which might be driven by multiple cycles of
GDP-for-GTP exchange on ARF1 and ARF1-mediated GTP hydrolysis, a process known
to be essential for loading of cargo molecules into COPI vesicles
(Nickel et al., 1998b;
Malsam et al., 1999
;
Pepperkok et al., 2000
).
However, it has to be pointed out that other models would explain
ARF-dependent coatomer binding to Golgi membranes equally well (see previous
section) and, therefore, at the present time this model remains entirely
speculative.
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A minimal machinery sufficient for COPI coat assembly, budding and uncoating |
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Liposome-derived COPI-coated vesicles are closely related to authentic
Golgi-derived COPI-coated vesicles with regard to density, size and
morphology. Strikingly, their formation does not depend on the lipid
composition of the donor bilayer. Liposomes composed of only egg yolk
phosphatidylcholine (PC) are sufficient for COPI vesicle formation, provided
that the p23 lipopeptide is present. These data suggest that specific lipids
are not essential for COPI recruitment to membranes. Interestingly, Spang et
al. have reported experimental conditions that promote formation of COPI
vesicles from protein-free liposomes, establishing a role for ARF-GTP and
coatomer in shaping the donor membrane
(Spang et al., 1998). The
lipid composition used in this work, however, is highly unlikely to exist in a
biological membrane. By contrast, p24 proteins not only bind to coatomer and
alter its conformation but also are abundant residents of the intermediate
compartment (IC) and the Golgi (Sohn et
al., 1996
; Rojo et al.,
1997
; Dominguez et al.,
1998
), the intracellular sites of COPI vesicle biogenesis
(Griffiths et al., 1995
;
Orci et al., 1997
). Therefore,
we propose that the minimal machinery for COPI vesicle formation consists of
p23/p24, ARF-GTP and coatomer (Fig.
2). While it is certainly possible that specific lipids (such as
acidic membrane lipids) might influence the rate of vesicle budding under
physiological conditions (De Camilli et
al., 1996
; Roth and Sternweis,
1997
), they do not appear to be essential components of the core
machinery required for the formation of COPI vesicles.
ARF-dependent GTP hydrolysis has been demonstrated to initiate COPI vesicle
uncoating (Tanigawa et al.,
1993). At the time, ARF1-specific GTPase-activating proteins were
not characterized in molecular terms. The first ARF1-GAP open reading frame
was identified by Cassel and colleagues followed by the characterization of
its domain structure (Cukierman et al.,
1995
). Interestingly, the catalytic domain of ARF1-GAP alone is
sufficient to uncoat synthetic COPI vesicles formed from
p23-lipopeptide-containing liposomes in the presence of ARF1-GTP and coatomer
(C. Reinhard and F. T. Wieland, unpublished). Although previous results
demonstrated a requirement for GTP hydrolysis in the uncoating reaction
(Tanigawa et al., 1993
), the
ability of the catalytic ARF-GAP domain to uncoat liposome-derived COPI
vesicles indicates that this activity is sufficient to convert coated vesicles
into naked vesicles. These data are consistent with Goldberg's finding that a
tripartite complex of ARF1, ARF1-GAP and coatomer controls ARF-mediated GTP
hydrolysis (Goldberg, 1999
).
As illustrated in Fig. 2, this
observation adds to our picture of the core machinery needed to mediate a full
round of COPI membrane recruitment, vesicular budding and coat removal, the
latter process being a prerequisite for fusion with target membranes of
transport intermediates.
In conclusion, available data support the view that p23 and p24 mediate
COPI recruitment and coat assembly under physiological conditions. As p24
proteins are localized to the early secretory pathway
(Sohn et al., 1996;
Rojo et al., 1997
;
Dominguez et al., 1998
), this
would also be a plausible explanation for the observation that COPI-coated
vesicle budding appears to be restricted to the IC and the Golgi
(Griffiths et al., 1995
;
Orci et al., 1997
). However,
this view has been challenged by yeast genetic experiments in which all known
p24 proteins were knocked out in a single strain. Severe transport phenotypes
could not be observed and the cells exhibited a morphologically normal
endomembrane system (Springer et al.,
2000
). These findings are even more surprising because a
p23-knockout in mice is embryonically lethal at the earliest possible stage
(Denzel et al., 2000
),
indicating that in mice p23 is essential. It is not yet clear why yeast cells
have access to some kind of alternative mechanism, or whether additional
factors cause lethality in a developing multicellular organism in the absence
of p23. One could address this problem by studying protein transport in
mammalian cells by employing RNA interference to temporarily inhibit p23 and
p24 protein expression.
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Future perspectives |
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Acknowledgments |
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