Practice question

BiP, also termed GRP78 encoded by HSPA5 gene, is abundant ER lumenal Hsp70 family member exhibiting both holdase and molecular motor activities. Structurally it comprises N-terminal nucleotide binding domain with actin-like ATPase fold and C-terminal substrate binding domain consisting of beta-sandwich that cradles extended peptide and alpha-helical lid that closes over cleft. In ATP-bound open state affinity low, allowing scanning of incoming nascent chains emerging through Sec61. DnaJ proteins like ERdj3/ERdj4 stimulate ATP hydrolysis, converting to ADP-bound closed conformation that clamps onto hydrophobic stretches typically five to seven residues enriched in branched aliphatic and aromatic side chains, preventing retrograde movement and aggregation. Nucleotide exchange factor Sil1 and Grp170 promote ADP release allowing substrate release for folding attempt. As ratchet during post-translational translocation, repetitive BiP binding prevents back-sliding providing directional force. Beyond translocation, BiP assists immunoglobulin folding, retains unassembled subunits, regulates UPR sensors IRE1, PERK, ATF6 via sequestration under resting conditions and releases upon stress. Signal cleavage or pore formation are not its activities, highlighting specialized chaperone motor function.