Membrane protein targeting to the MVB/lysosome.

Abstract

Cells sense and respond to their environment largely through the function of receptors, transporters, and channels within the plasma membrane. For cells to appropriately respond to environmental cues, they must maintain the proper protein complement at the cell surface. This involves both delivering proteins to the cell surface and removing them when necessary. The multivesicular body (MVB) sorting reaction within the endocytic pathway provides an important cellular mechanism for terminating the function of integral membrane proteins destined for degradation within the lysosome (reviewed in 1–3). The MVB sorting machinery recognizes a subset of endocytic cargoes and concentrates them into regions within the endosomal membrane. These sections then bud as small vesicles into the lumen of the endosome, giving the endosome a multivesicular appearance by electron microscopy. Subsequent fusion of the MVB with the lysosome delivers these intralumenal vesicles to the hydrolytic environment of the lysosome, where the lipid and protein contents of the vesicles are degraded (reviewed in 4). The topology of membrane invagination into the endosomal lumen (exvagination from the cytosol) during MVB sorting is similar to the budding process whereby enveloped viruses, including HIV-1 and Ebola virus, egress from the cell (Figure 1). Viral structural proteins, such as Gag from HIV-1 and VP40 from Ebola virus, utilize the MVB sorting machinery to bud either from the plasma membrane or into an intracellular compartment for subsequent release from the cell (reviewed in 5,6). Therefore, the MVB sorting machinery is important both for viral replication as well as for lysosomal delivery of endocytosed proteins. In addition, a number of recent studies have shown a role for the MVB sorting machinery in late steps of cytokinesis 7–13, emphasizing the importance of this machinery for diverse cellular processes of similar membrane topology. Figure 1 Roles for cellular machinery in MVB sorting and viral particle formation MVB sorting is conserved throughout eukaryotes, and studies in both yeast and mammalian systems have identified a series of trans-acting factors that mediate this reaction (reviewed in 2,14,15). The endosomal sorting complexes required for transport (ESCRTs) and associated proteins constitute the majority of this machinery. Cargo recognition is mediated by interactions with the Vps27/Hrs-Hse1/STAM complex as well as with ESCRT-I (Vps23/Tsg101, Vps28, Vps37 and Mvb12) 16–20. ESCRT-II (Vps22, Vps25, Vps36) appears to function downstream or in parallel to ESCRT-I, and is also believed to interact with cargo based on ubiquitin-binding domain activity within Vps36 21–25. ESCRT-II additionally serves to facilitate the recruitment and assembly of ESCRT-III subunits (Vps20/CHMP6, Snf7/CHMP4, Vps2/CHMP2, Vps24/CHMP3, Did2/CHMP1, Vps60/CHMP5) into a polymer 25–28. Interactions between ESCRT-III and the Vps4-Vta1 complex then promote disassembly of ESCRT-III and may be linked to the completion of intralumenal vesicle (ILV) formation 29–35. The concerted actions of these multimeric complexes serve to concentrate and deliver MVB cargoes into ILVs of the endosome for their eventual degradation within the lysosome. While lysosomal degradation provides a mechanism for the long-term attenuation of transmembrane proteins (such as cell surface receptors, transporters, and channels), sorting into the MVB appears to abrogate their functions prior to degradation. Sorting activated receptors into the MVB sequesters their cytoplasmic kinase domains into the endosomal lumen and away from cytoplasmic substrates, effectively terminating signaling from activated receptors. A well-studied example illustrating the importance of this reaction is the sorting of activated epidermal growth factor receptor (EGFR) (reviewed in 36,37). Defects that prevent EGFR sorting into intralumenal vesicles are oncogenic due to receptor hyperactivity, and this oncogenic activity is attributed to deficiencies in both receptor sequestration and lysosomal degradation (reviewed in 38,39). While MVB sorting can have an important impact on signaling, signaling can also impact MVB sorting. EGFR delivery into the MVB pathway is accelerated in response to EGF stimulation; moreover, EGF stimulation actually promotes MVB biogenesis 40,41. The Hrs-STAM complex is phosphorylated after stimulation of receptor tyrosine kinases 42–44, and this phosphorylation impacts machinery levels and receptor degradation 45. Therefore, it is important to understand the mechanisms by which signaling and MVB sorting impact each other and how flux through the MVB pathway is regulated.