How ubiquitin functions with ESCRTs

Abstract

The endosomal-sorting complex required for transport (ESCRT) apparatus has multiple ubiquitin (Ub)-binding domains and participates in a wide variety of cellular processes. Many of these ESCRT-dependent processes are keenly regulated by Ub, which serves as a lysosomal-sorting signal for membrane proteins targeted into multivesicular bodies (MVBs) and which may serve as a mediator of viral budding from the cell surface. Hints that both ESCRTs and Ub work together in the processes such as cytokinesis, transcription and autophagy are beginning to emerge. Here, we explore the relationship between ESCRTs and Ub in MVB sorting and viral budding.

Figure 1

Modular arrangement of ESCRTs for sorting Ubiquitinated cargo into Intralumenal endocytic vesicles. The ESCRT apparatus contains several Ub-binding domains (Red). These are distributed amongst ESCRT-0 and a wider set of proteins that might serve a similar parallel function as ESCRT-0. Together, these proteins recognize ubiquitinated membrane cargo and deliver it to the ESCRT-I/II supercomplex which coordinates cargo sorting and incorporation into intralumenal vesicles with the ESCRT-III apparatus used for completing intralumenal vesicle biogenesis. The individual components of the ESCRT-I and II are listed along with their mammalian counterparts. For simplicity, the various isoforms of particular mammalian ESCRT subunits are not designated. Once cargo is sorted and vesicles are formed, a polymer of ESCRT-III subunits forms and facilitates scission of intralumenal vesicles. The Doa4 deubiquitinating enzyme is recruited to the ESCRT-III complex by interacting with Bro1. In mammalian cells, AMSH and USP8 can also interact with ESCRT-III, although that interaction is not dependent on being bridged by the Bro1 homolog, Alix. The Vps4 AAA ATPase complex is required for de-polymerizing the ESCRT-III complex.

Figure 2

Viral Gag proteins contain late domains that interact with different cellular host proteins to promote viral egress. The schematic depicts a Gag protein with an N-terminal membrane association domain and a late domain containing various interaction sites. PTAP-motifs recruit ESCRT-I through interactions with the UEV-domain within Tsg101, LYPL/LxxLY-motifs recruit ALIX through interactions with its V-domain, and PPxY-motifs recruit Nedd4-family of Ub ligases. Gag proteins are ubiquitinated, which may promote association with ESCRTs; however, other cellular proteins may be functional targets of ubiquitination as well. Ubiquitin may be involved as a recruitment pathway by allowing TSG101, ALIX, or another ESCRT to bind Gag proteins. Alternatively, Ub may regulate aspects of ESCRT function. Intriguingly, the function of late domains along with the ESCRT pathway can be circumvented with a Gag containing a domain that allows for higher order oligomerization (176-178). These data indicate that the ESCRT dependent process may provide a similar function. Pictured at the tope left is a stochastic model whereby several alternate ubiquitinated proteins could interact with any number of Ub-binding ESCRTs or ESCRT-associated proteins to allow for viral budding.