Practice question

Endoplasmic reticulum lumen is specialized folding compartment with oxidizing redox potential, high calcium millimolar and abundant chaperones distinguishing it from reducing cytosol. Newly translocated polypeptides entering via Sec61 encounter BiP also called GRP78, luminal Hsp70 family member binding transiently to exposed hydrophobic segments in ATP-regulated cycle where ATP binding opens substrate binding domain, hydrolysis closes it trapping unfolded chain preventing aggregation and promoting correct disassembly of aggregates. Cooperation with co-chaperone ERdj proteins stimulates ATPase, nucleotide exchange factor GRP170 releases. Additional folding assistants include calnexin calreticulin lectins sensing N-glycans, protein disulfide isomerase forming disulfides. ATP synthase resides in mitochondrial cristae generating ATP via proton motive force, ribosomes attach cytosolic face not inside, histone acetyltransferases act in nucleus. Unfolded protein response upregulates BiP transcription via ATF6 and IRE1 pathways when misfolded load increases, restoring folding capacity. Thus lumen proteostasis depends on chaperone inventory. Integration with cell cycle kinases, calcium signaling and mechanical cues ensures coordinated remodeling during growth, migration and differentiation.