Retromer

Abstract

The retromer is a heteropentameric complex that associates with the cytosolic face of endosomes and mediates retrograde transport of transmembrane cargo from endosomes to the trans-Golgi network. The mammalian retromer complex comprises a sorting nexin dimer composed of a still undefined combination of SNX1, SNX2, SNX5 and SNX6, and a cargo-recognition trimer composed of Vps26, Vps29 and Vps35. The SNX subunits contain PX and BAR domains that allow binding to PI(3)P enriched, highly curved membranes of endosomal vesicles and tubules, while Vps26, Vps29 and Vps35 have arrestin, phosphoesterase and alpha-solenoid folds, respectively. Recent studies have implicated retromer in a broad range of physiological, developmental and pathological processes, underscoring the critical nature of retrograde transport mediated by this complex.

Figure 1

Sorting of acid hydrolases mediated by mannose 6-phosphate receptors. The scheme represents the sorting of acid hydrolases by transmembrane, mannose 6-phosphate receptors (MPR) that cycle between the trans-Golgi network (TGN) and endosomes. Coats containing clathrin, GGA and AP-1 mediate exit of hydrolase-MPR complexes from the TGN, whereas retromer mediates the retrieval of unoccupied MPR from endosomes. Sequestration of MPR into recycling tubules by retromer prevents their delivery to the degradative, multivesicular body (MVB) pathway by the clathrin-ESCRT machinery. A fraction of MPR are transported from the TGN to the plasma membrane, after which they are internalized by coats containing clathrin and AP-2, joining the pool of recycling receptors in endosomes. See text for additional details and references. TEN: tubular endosomal network; H⁺ represents the acidic pH of the endosome lumen.

Figure 2

Domain organization of mammalian retromer subunits. Boxes indicate folded domains and lines largely disordered sequences. Number of amino acids and molecular masses of each subunit are indicated. The functions of each domain are denoted on top. All subunits occur as multiple splice forms, only one of which is represented here. The exact subunit composition of the mammalian retromer complex is not known with certainty, although a current view is that it comprises a dimer of SNX1/2 with SNX5/6, and a trimer composed of one copy each of Vps26A/B, Vps29 and Vps35.

Figure 3

Structure of retromer. Retromer subunits are colored as in Fig. 2. The crystal structure of human Vps26A is shown in cyan (PDB 2FAU; [26*]), and those of human Vps29 and Vps35 are shown in green and red, respectively (PDB 2R17; [28**]). Residues surrounding the inactive metallophosphoesterase site of VPS29 are shown in a stick model. The structure of the SNX dimer is represented by the crystal structure of the SNX9 PX-BAR unit [24*]. The entire SNX dimer model is shown in purple, with the PI(3)P shown in a stick model. The structures are shown relative to a hypothetical model for the retromer coat bound to a membrane tubule (modified from ref. [28**]).

Figure 4

Presence of retromer on vacuolar (E) and tubular (arrows) endosomes. A. Imaging of the cytoplasm of a live cell expressing Vps29 tagged with the yellow fluorescent protein (Vps29-YFP) A video showing the dynamics of Vps29-YFP-containing endosomes can be seen at http://cellimages.ascb.org/cdm4/item_viewer.php?CISOROOT=/p4041coll12&CISOPTR=258&CISOBOX=1&REC=4. Image and video are courtesy of Gonzalo Mardones, NICHD, NIH. B. Immunoelectron microscopy of the localization of endogenous Vps26 labeled with 10 nm gold particles (reproduced with permission from ref. [6]). Arrowhead shows a flat, electron dense coat characteristic of coated endosomes. Bar: 100 nm.