Controversy surrounds the cellular role of the Bcl-2 family protein Bok. On one hand, it has been shown that all endogenous Bok is bound to inositol 1,4,5-trisphosphate receptors (IP3Rs), while other data suggest that Bok can act as a pro-apoptotic mitochondrial outer membrane permeabilization mediator, apparently kept at very low and non-apoptotic levels by efficient proteasome-mediated degradation. Here we show that 1) endogenous Bok is expressed at readily-detectable levels in key cultured cells (e.g., mouse embryonic fibroblasts and HCT116 cells) and is not constitutively degraded by the proteasome, 2) proteasome inhibitor-induced apoptosis is not mediated by Bok, 3) endogenous Bok expression level is critically dependent on the presence of IP3Rs, 4) endogenous Bok is rapidly degraded by the ubiquitin-proteasome pathway in the absence of IP3Rs at the endoplasmic reticulum membrane, and 5) charged residues in the transmembrane region of Bok affect its stability, ability to interact with Mcl-1, and pro-apoptotic activity when over-expressed. Overall, these data indicate that endogenous Bok levels are not governed by proteasomal activity (except when IP3Rs are deleted) and that while endogenous Bok plays little or no role in apoptotic signaling, exogenous Bok can mediate apoptosis in a manner dependent on its transmembrane domain.
Coronaviruses (CoV), including SARS-CoV-2, modulate host proteostasis pathways during infection through activation of stress-responsive signaling pathways such as the Unfolded Protein Response (UPR). The UPR regulates protein translation, increases protein folding capacity and enhances endoplasmic reticulum (ER) biogenesis to alleviate ER stress caused by accumulation of misfolded proteins. CoVs depend on host machinery to generate large amounts of viral protein and manipulate ER-derived membranes to form double-membrane vesicles (DMVs), which serve as replication sites, making the UPR a key host pathway for CoVs to hijack. Despite the importance of CoV nonstructural proteins (nsps) in mediating replication, little is known about the role of nsps in modulating the UPR. We characterized the impact of SARS-CoV-2 nsp4, which is a key driver of DMV formation, on the UPR using quantitative proteomics. We find nsp4 preferentially activates the ATF6 and PERK branches of the UPR. Previously, we found an N-terminal truncation of nsp3 (nsp3.1) can suppress pharmacological activation of the ATF6 pathway. To determine how nsp3.1 and nsp4 might tune the UPR in concert, both proteins were co-expressed demonstrating that nsp3.1 does not suppress nsp4-mediated ATF6 activation but does suppress PERK activation. A meta-analysis of SARS-CoV-2 infection proteomics data reveals a time-dependent activation of PERK protein markers early in infection, which subsequently fades. This temporal regulation suggests a role for nsps tuning the PERK pathway to attenuate host translation beneficial for viral replication while avoiding later apoptotic signaling caused by chronic PERK activation. This work furthers our understanding of CoV-host proteostasis interactions and identifies potential areas to target for anti-viral therapies.
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