BAP31 and BiP are essential for dislocation of SV40 from the endoplasmic reticulum to the cytosol

Abstract

How non-enveloped viruses overcome host cell membranes is poorly understood. Here, we show that after endocytosis and transport to the endoplasmic reticulum (ER), but before crossing the ER membrane to the cytosol, incoming simian virus 40 particles are structurally remodelled leading to exposure of the amino-terminal sequence of the minor viral protein VP2. These hydrophobic sequences anchor the virus to membranes. A negatively charged residue, Glu 17, in the α-helical, membrane-embedded peptide is essential for infection, most likely by introducing an 'irregularity' recognized by the ER-associated degradation (ERAD) system for membrane proteins. Using a siRNA-mediated screen, the lumenal chaperone BiP and the ER-membrane protein BAP31 (both involved in ERAD) were identified as being essential for infection. They co-localized with the virus in discrete foci and promoted its ER-to-cytosol dislocation. Virus-like particles devoid of VP2 failed to cross the membrane. The results demonstrated that ERAD-factors assist virus transport across the ER membrane.

CD4 and BST-2/Tetherin Proteins Retro-translocate from Endoplasmic Reticulum to Cytosol as Partially Folded and Multimeric Molecules

“…The ER-associated degradation (ERAD) pathway operates as a quality control mechanism to dispose of such unwanted molecules through proteasomal degradation (3). Retro-translocation, however, also operates in cases of exit from the ER not directly involved in ERAD, as observed for calreticulin (4), SV40 virus (5), and some bacterial and plant toxins (6 -9).…”

How Viruses Use the Endoplasmic Reticulum for Entry, Replication, and Assembly

“…For some Pys, a dimeric redox-inactive PDI protein called ERp29 untangles the VP1 carboxy-terminal arms (Magnuson et al 2005;Rainey-Barger et al 2007a,b;Nelson et al 2012) exposing hydrophobic VP2 and VP3 to generate a hydrophobic viral particle ( Fig. 2A, step 1b) (Magnuson et al 2005;Rainey-Barger et al 2007a;Geiger et al 2011). The hydrophobic virus is likely maintained in a soluble state by binding to the Hsp70 chaperone BiP ( Fig.…”

“…The hydrophobic virus is likely maintained in a soluble state by binding to the Hsp70 chaperone BiP ( Fig. 2A, step 1c) (Geiger et al 2011;Goodwin et al 2011) in a reaction regulated by the J-domain containing cochaperone ERdj3 (Goodwin et al 2011). The J domain typically stimulates Hsp70's ATPase activity to control Hsp70-substrate interaction (Kampinga and Craig 2010).…”

“…2A, step 2) (Daniels et al 2006;Rainey-Barger et al 2007b;Geiger et al 2011). At this juncture, ERAD membrane components including Derlin-1 (Schelhaas et al 2007;Jiang et al 2009b), Derlin-2 (Lilley et al 2006), Sel1L (Schelhaas et al 2007), RMA1 (Geiger et al 2011), and BAP29/BAP31 (Geiger et al 2011), have been proposed to facilitate Py's exit to the cytosol. However, as none of them have been shown to interact with Py physically, their actions on the virus may be transient or indirect.…”

“…How this step is accomplished is the least understood. The cytosolic ATPase p97, which normally extracts misfolded cellular proteins into the cytosol during ERAD (Ye et al 2001), plays only a minor role in releasing Py into the cytosol (Geiger et al 2011). Furthermore, although there is evidence implicating the proteasome at this stage (Schelhaas et al 2007;Jiang et al 2009b;, it is unlikely to play a direct role.…”