Which of the following is not a function of the rough ER?
Rough ER domains studded with polysomes perform cotranslational folding and modification of secretory and membrane proteins. As nascent chain emerges into lumen via Sec61, protein disulfide isomerase catalyzes oxidative formation of disulfide bonds favored by high ratio of oxidized to reduced glutathione, peptidyl-prolyl isomerases accelerate isomerization, and oligosaccharyltransferase attaches N-linked glycans to asparagine within N-X-S/T sequons. Chaperones BiP and calnexin monitor folding, retaining immature proteins preventing aggregation. N-linked glycosylation and disulfide formation serve as folding sensors. Conversely lipid biosynthesis including phosphatidylcholine synthesis via Kennedy pathway, triglyceride, cholesterol and steroid hormone precursors requires enzymes such as HMG-CoA reductase, acyltransferases and cytochrome P450 that localize to smooth ER lacking ribosomes, allowing hydrophobic substrates access without nascent chain interference. Thus lipid synthesis is characteristic smooth not rough ER function, illustrating compartmentalization optimizing protein quality control in rough domains while dedicating smooth tubules to lipid anabolism. Integration with cell cycle kinases, calcium signaling and mechanical cues ensures coordinated remodeling during growth, migration and differentiation.
Ref: Alberts Ch 12; rough ER folding disulfide N-glycosylation BiP calnexin; lipid synthesis smooth ER.