The Fifth Adaptor Protein Complex
Citation: Hirst J, Barlow LD, Francisco GC, Sahlender DA, Seaman MNJ, et al. (
The Fifth Adaptor Protein Complex
Jennifer Hirst 0
Lael D. Barlow 0
Gabriel Casey Francisco 0
Daniela A. Sahlender 0
Matthew N. J. Seaman 0
Joel B. Dacks 0
Margaret S. Robinson 0
Sandra L. Schmid, The Scripps Research Institute, United States of America
0 1 University of Cambridge, Cambridge Institute for Medical Research, Cambridge, United Kingdom, 2 Department of Cell Biology, University of Alberta , Edmonton , Canada
Adaptor protein (AP) complexes sort cargo into vesicles for transport from one membrane compartment of the cell to another. Four distinct AP complexes have been identified, which are present in most eukaryotes. We report the existence of a fifth AP complex, AP-5. Tagged AP-5 localises to a late endosomal compartment in HeLa cells. AP-5 does not associate with clathrin and is insensitive to brefeldin A. Knocking down AP-5 subunits interferes with the trafficking of the cationindependent mannose 6-phosphate receptor and causes the cell to form swollen endosomal structures with emanating tubules. AP-5 subunits can be found in all five eukaryotic supergroups, but they have been co-ordinately lost in many organisms. Concatenated phylogenetic analysis provides robust resolution, for the first time, into the evolutionary order of emergence of the adaptor subunit families, showing AP-3 as the basal complex, followed by AP-5, AP-4, and AP-1 and AP-2. Thus, AP-5 is an evolutionarily ancient complex, which is involved in endosomal sorting, and which has links with hereditary spastic paraplegia.
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For many years, it has been assumed that there are four, and
only four, adaptor protein (AP) complexes. The first two AP
complexes to be identified, AP-1 and AP-2, sort cargo proteins into
clathrin-coated vesicles (CCVs). Both AP-1 and AP-2 are
heterotetramers, consisting of two large subunits, sometimes called
adaptins (c and b1 in AP-1; a and b2 in AP-2); a medium-sized
subunit (m1 or m2); and a small subunit (s1 or s2) (Figure 1a and
b). The AP-3 and AP-4 complexes were discovered by searching
databases for homologues of the AP-1 and AP-2 subunits. AP-3
and AP-4 are also heterotetramers, made up of d, b3, m3, and s3
subunits, and of e, b4, m4, and s4 subunits, respectively. However,
unlike AP-1 and AP-2, they appear to be able to work without
clathrin (reviewed in [1,2]).
Each of the AP complexes has a distinct localisation and function.
AP-1 is localised to tubular endosomes and/or the trans-Golgi
network (TGN) and is involved in trafficking between the two
organelles, although there is still some uncertainty about the
direction [3]. AP-2, the most thoroughly characterised of the four
complexes, facilitates clathrin-mediated endocytosis [4]. AP-3
traffics cargo from tubular endosomes to late endosomes, lysosomes,
and related organelles, while AP-4 has recently been shown to traffic
the amyloid precursor protein from the TGN to endosomes [5].
Thus, all of the AP complexes are involved in post-Golgi trafficking
pathways (Figure 1c). In animals, gene knockouts of AP-1 or AP-2
subunits are embryonic lethal [6]. However, animals can survive
without AP-3 or AP-4, and mutations in the two complexes in
humans have been shown to cause Hermansky Pudlak syndrome [7]
and neurological disorders [810], respectively.
The degree of identity between the related sets of subunits in the
four AP complexes is generally in the range of 20%40%. Another
more distantly related heterotetrameric complex is the F
subcomplex of the COPI coat (F-COPI), which acts in an earlier
pathway, packaging cargo into vesicles for retrograde trafficking
from the Golgi apparatus to the ER [1]. F-COPI consists of the
large subunits c-COP and b-COP, the medium subunit d-COP,
and the small subunit f-COP. There is also ancient homology
between all of the large subunits and between the small subunits
and the N-terminal domains of the medium subunits [11].
Although in these cases the sequence identities are no more than
,10%, the relationship is detectable by sensitive homology
searching algorithms, and structural studies show that the proteins
adopt very similar folds [12,13].
This relationship has led to the hypothesis that both complexes
evolved from an ancestral heterodimer, consisting of a large chain
(the ancestor of all of the large subunits) and a small chain (the
ancestor of both medium and small subunits) [1,11]. This
hypothesis is supported by the finding that there are strong
interactions between the c/a/d/e-adaptin/c-COP large subunits
and the s1-4-adaptin/f-COP small subunits and between the
b14-adaptin/b-COP large subunits and the N-terminal domains of
the m1-4-adaptin/d-COP medium subunits [14,15]. A further
round of gene duplication would have given rise to the F-COPI
complex and the proto-AP complex [1,6].
All of these events must have occurred over one billion years
ago, because subunits of all four of the AP complexes, as well as
COPI, can be fo (...truncated)