Observing the nonvectorial yet cotranslational folding of a multidomain protein, LDL receptor, in the ER of mammalian cells

SignificanceMultidomain proteins constitute more than two-thirds of all proteome. To decrease the risk of misfolding, these proteins generally fold vectorially as each domain emerges from the ribosome or protein translocation channel. Interestingly, however, some cell surface multidomain proteins including the low-density lipoprotein receptor (LDLR) fold in the ER nonvectorially via intermediates with nonnative disulfides. Shuffling of nonnative disulfides to native ones, a key step in their folding, has been thought to proceed slowly after synthesis. Here, we find that disulfide shuffling in LDLR occurs at a specific timing during synthesis in a manner depending on a downstream region of the polypeptide. Thus, nonvectorial folding of multidomain proteins in the endoplasmic reticulum (ER) may be more coordinated and elaborated than thought.