Divergent retroviral late-budding domains recruit vacuolar protein sorting factors by using alternative adaptor proteins

TLDR

Enveloped viruses rely on late‑budding domains (PTAP, PPXY, YPDL) to recruit host ESCRT machinery, but the downstream steps and the role of YPDL/PPXY domains, which do not require ESCRT‑I, remain poorly understood. The study aims to demonstrate that ESCRT‑I and other class E VPS factors are connected through a complex network of protein–protein interactions. This network is mediated by AIP‑1/ALIX, which bridges ESCRT‑I to ESCRT‑III, linking the late‑budding machinery to the class E VPS pathway. Fusion‑protein expression of ESCRT‑III components disrupts budding across all L‑domain types, and EIAV’s YPDL domain specifically depends on AIP‑1/ALIX, as shown by inhibition with truncated protein or siRNA, indicating that L‑domains hijack distinct class E VPS factors—some essential for all types, others acting as adaptors—to drive viral release.

Abstract

The release of enveloped viruses from infected cells often requires a virally encoded activity, termed a late-budding domain (L domain), encoded by essential PTAP, PPXY, or YPDL sequence motifs. PTAP-type L domains recruit one of three endosomal sorting complexes required for transport (ESCRT-I). However, subsequent events in viral budding are poorly defined, and neither YPDL nor PPXY-type L domains require ESCRT-I. Here, we show that ESCRT-I and other class E vacuolar protein sorting (VPS) factors are linked by a complex series of protein–protein interactions. In particular, interactions between ESCRT-I and ESCRT-III are bridged by AIP-1/ALIX, a mammalian orthologue of the yeast class E VPS factor, Bro1. Expression of certain ESCRT-III components as fusion proteins induces a late budding defect that afflicts all three L-domain types, suggesting that ESCRT-III integrity is required in a general manner. Notably, the prototype YPDL-type L domain encoded by equine infectious anemia virus (EIAV) acts by recruiting AIP-1/ALIX and expression of a truncated form of AIP-1/ALIX or small interfering RNA-induced AIP-1/ALIX depletion specifically inhibits EIAV YPDL-type L-domain function. Overall, these findings indicate that L domains subvert a subset of class E VPS factors to mediate viral budding, some of which are required for each of the L-domain types, whereas others apparently act as adaptors to physically link specific L-domain types to the class E VPS machinery.