The retromer complex – endosomal protein recycling and beyond
Matthew N. J. Seaman
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University of Cambridge, Cambridge Institute for Medical Research, and Department of Clinical Biochemistry, Addenbrooke's Hospital
, Hills Road, Cambridge CB2 0XY,
UK
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to-Golgi retrieval of the carboxy peptidase Y (CPY) receptor
Vps10p. The complex, as characterised in yeast, comprises five
proteins that are all encoded by vacuole protein sorting (VPS)
genes (Seaman et al., 1997; Seaman et al., 1998). The yeast
heteropentameric retromer complex can be biochemically and
phenotypically dissected into two subcomplexes: a trimer of
Vps35p, Vps29p and Vps26p, which mediates cargo selection, and
a dimer of Vps5p with Vps17p. The Vps5 and Vps17 proteins are
members of the sorting nexin (Snx) family and are believed to
mediate tubule or vesicle formation through their C-terminal
BinAmphiphysin-Rvs (BAR) domains and an intrinsic self-assembly
activity (Nothwehr and Hindes, 1997; Horazdovsky et al., 1997;
Kurten et al., 2001; Peter et al., 2004). Vps5p, Vps17p and related
proteins are frequently described as Snx-BAR proteins to
distinguish them from other members of the Snx family
(reviewed in Attar and Cullen, 2010).
In the 14 years since retromer was identified, much has been
learned about how retromer functions in endosomal protein
sorting. Retromer subunits are highly conserved, and in higher
eukaryotes, such as mammalian cells, the cargo selective trimer is
essentially identical to the yeast complex. In addition, similar to
in yeast, the Snx-BAR component is a dimer, whereby a
Vps5like protein associates with an orthologue of Vps17p. For
example, in mammalian cells the homologues of Vps5p, namely
Snx1 or Snx2 dimerise with a Vps17p orthologue, specifically
either Snx5 or Snx6 (Wassmer et al., 2007).
There are, however, important differences between the
prototypical retromer in yeast and retromer in higher eukaryotes.
A key difference is that, in mammalian cells, retromer is not a
The identification of retromer
The retromer complex was first identified in the yeast
Saccharomyces cerevisiae and was shown to mediate
endosomestable heteropentamer but a much more transient association of the
cargo-selective trimer (Vps35, Vps26 and Vps29) and the Snx
components (Snx1 or Snx2 with Snx5 or Snx6) (Swarbrick et al.,
2011). From studies in various model systems including
mammalian cells, there are now a growing number of reports of
cargo proteins and processes that utilise only one of the retromer
subcomplexes without requiring the function of the other (Nisar
et al., 2010; Prosser et al., 2010). This difference means that
when discussing retromer in mammalian cells or other higher
eukaryotes, it is important to specify which functional unit of
retromer is being referred to, the cargo-selective trimer or the
membrane-bending Snx-BAR dimer. It is also apparent that many
of the retromer accessory proteins identified from studies in higher
eukaryotes are not conserved in yeast; the implications of this
observation are discussed below.
Recruitment of retromer to the membrane
Recruitment of the retromer subcomplexes to the endosomal
membrane precedes or occurs concomitantly with the selection of
cargo proteins a process mediated by the Vps35Vps29Vps26
trimer. Simultaneously, the Snx-BAR dimer assembles to
promote formation of endosomal membrane tubules into which
cargo proteins are sorted. Following tubule formation, the
retromer proteins then dissociate from the membrane so that
further rounds of cargo sorting can occur. Recruitment of
the retromer subcomplexes to the endosome is therefore a key
element of the regulation of retromer function.
Recruitment of the Snx dimer
The recruitment of the Snx-BAR dimer is mediated through
binding to phosphatidylinositol (PtdIns) 3-phosphate [PtdIns(3)P]
by the Phox homology (PX) domain, which is present in each Snx
protein (Cozier et al., 2002). Production of PtdIns(3)P in both
yeast and mammalian cells occurs as a result of the activity of the
phosphoinositide 3-kinase (PI3K) Vps34 (Stack et al., 1993).
Vps34 is itself regulated by the Vps30 protein in yeast or its
metazoan homologue Beclin1 (Burda et al., 2002; Ruck et al.,
2011). Studies in mammalian cells have shown that RNA
interference (RNAi)-meditated loss of the cargo-selective
complex does not prevent the recruitment of the Snx-BAR
dimer, demonstrating that the Snx-BAR dimer associates with the
membrane independently of the cargo-selective complex
(Seaman, 2004; Arighi et al., 2004).
Recruitment of the cargo-selective complex
How the cargo-selective complex is recruited to the membrane is
an important question because its recruitment is necessary for
cargo selection. Unlike the Snx-BAR dimer, none of the proteins
that comprise the cargo-selective complex contain a known
lipidbinding domain. It has now been shown that the small GTPase
Rab7a is required for recruitment of the cargo-selective trimer
(Rojas et al., 2008; Seaman et al., 2009). The association between
the cargo-selective complex an (...truncated)