Orchestration of SARS-CoV-2 Nsp4 and host cell ESCRT proteins induces morphological changes of the endoplasmic reticulum.

Upon entry into the host cell, the nonstructural proteins 3, 4, and 6 (Nsp3, Nsp 4, and Nsp6) of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) facilitate the formation of double-membrane vesicles (DMVs) through extensive rearrangement of the host cell endoplasmic reticulum (ER) to replicate the viral genome and translate viral proteins. To dissect the functional roles of each Nsp and the molecular mechanisms underlying the ER changes, we exploited both yeast Saccharomyces cerevisiae and human cell experimental systems. Our results demonstrate that Nsp4 alone is sufficient to induce ER structural changes. Nsp4 expression led to robust activation of both the unfolded protein response (UPR) and the ER surveillance (ERSU) cell cycle checkpoint, resulting in cortical ER inheritance block and septin ring mislocalization. Interestingly, these ER morphological changes occurred independently of the canonical UPR and ERSU components but were mediated by the endosomal sorting complex for transport (ESCRT) proteins Vps4 and Vps24 in yeast. Similarly, ER structural changes occurred in human cells upon Nsp4 expression, providing a basis for a minimal experimental system for testing the involvement of human ESCRT proteins and ultimately advancing our understanding of DMV formation.