Practice question

Maintaining ER proteome against continuous leakage via bulk flow into Golgi relies active retrieval preventing depletion resident chaperones folding enzymes. ER membrane proteins expose C-terminal dilysine motif KKXX or KXKXX with two lysines at -3 -4 positions relative extreme C-terminus recognized directly WD40 beta-propeller domains COPI subunits alpha-COP beta prime-COP binding dilysine pH independent. Soluble ER luminal residents like BiP HSPA5 GRP94 PDI calreticulin bear C-terminal KDEL; after escape bind KDEL receptor family Erd2 homologs KDELR1-3 seven transmembrane proteins concentrated cis-Golgi ERGIC mildly acidic pH favors high-affinity binding. Receptor-cargo complex displays cytosolic dilysine-like retrieval signal receptor tail directly interacts COPI coatomer causing concentration COPI coated vesicles budding Golgi rims. COPI assembly triggered small GTPase ARF1-GTP produced GBF1 GEF generating membrane curvature. Vesicles move retrogradely ERGIC ER where de-coating SNARE-mediated fusion via syntaxin18 Use1 BNIP1 Sec22b complex returns content ER lumen membrane. Upon ER arrival neutral pH causes KDEL release. COPII Sec23-24 performs anterograde export, clathrin mediates TGN-endosome trafficking endocytosis dynamin performs scission. Brefeldin A pharmacologically inhibits ARF GEF blocking COPI formation demonstrating indispensability retrograde ER protein return Golgi ER quality control and homeostasis.