ARRDC1 as a mediator of microvesicle budding.

E ukaryotic cells critically depend on the formation, budding, and scission of membrane-bounded vesicles for many key processes: internalization of cell surface receptors, delivery of cargo proteins to multivesicular bodies (MVBs) and lysosomes for degradation, transport of newly synthesized proteins between intracellular organelles and their delivery to the plasma membrane, and release of microvesicles that function as vehicles for intercellular communication. Although distinct families of proteins and multiprotein complexes have been described that mediate these diverse events in the cell, the field of membrane trafficking remains an active area of investigation. In PNAS, the work by Nabhan et al. (1) describes a role for the arrestin domain-containing protein ARRDC1 in the generation of microvesicles—termed ARRDC1-mediated microvesicles (ARMMs)—that bud directly from the plasma membrane (PM). Arrestins are well-established regulators of cell signaling pathways, and they act to recruit factors that promote the endocytosis of PM receptors (2). Arrestins can interact directly with ubiquitin ligases, thereby stimulating the ubiquitination of PM receptors and in some cases, the arrestin itself. For example, the work by Lin et al. (3) characterized a family of arrestin-related proteins that recruits the yeast Nedd4-like ubiquitin ligase, Rsp5, to down-regulate the expression of amino acid transporters. Rsp5 was recruited by a PPxY-related motif in the arrestin proteins (3). Several recent studies have uncovered an intriguing connection between arrestin domain-containing proteins and components of the endosomal sorting complex required for transport (ESCRT) machinery (3–6). This machinery, which is composed of four multiprotein complexes (ESCRT-0 to ESCRT-III) (Fig. 1A), is required for the delivery of ubiquitinated cargo into vesicles that bud into late endosomes to form MVBs (7). ESCRT machinery also plays a key role in the budding of many enveloped viruses, including HIV-1 and other retroviruses (8). Recruitment of ESCRT machinery by retroviral Gag proteins (the viral structural proteins that drive particle assembly and release) requires the presence of so-called late domains in Gag; PTAP, PPxY, and YPxL late domains recruit the Tsg101 subunit of ESCRT-I, Nedd4 family ubiquitin ligases, and the ESCRT-associated factor Alix, respectively (9). A direct interaction was previously identified between a yeast arrestin-related protein, Rim8, and the yeast homolog of Tsg101, Vps23 (4). This interaction was mediated by binding between monoubiquitinated Rim8 and the ubiquitin E2 variant (UEV) domain of Vps23. Rim8 ubiquitination was dependent on the ubiquitin ligase, Rsp5. Thus, Rim8 functions as an arrestinrelated trafficking adaptor to recruit the ESCRT-I complex. Interactions between arrestin-related proteins and ESCRT components—specifically Tsg101 and Alix—have also been described in mammalian cells (6). These studies served as