Investigation of folding and degradation of mutant proteins synthesized in semipermeabilized cells: Protocol

The endoplasmic reticulum (ER) is the site where most secretory proteins acquire their native conformation and gain access to the secretory pathway, and the cell surface. Proteins entering the secretory pathway are translocated across or inserted into the ER membrane either co-translationally or post-translationally through an aqueous pore in the ER membrane called the translocon (1). The emerging polypeptide chains may then interact with molecular chaperones to ensure their correct folding and assembly (2). Covalent modification of the polypeptide chain by the formation of native inter- and intrachain disulphide bonds stabilizes folded protein domains and cross-links subunits associated with oligomeric complexes. The ability of the ER-molecular chaperones and folding enzymes to recognize and bind to non-native substrates retains these proteins within the ER until they have reached their native state (3). The dissociation of the fully folded substrates from the ER chaperones facilitates the transport process, resulting in exit of the native protein from the ER. Therefore, the “quality-control” system of the ER allows export of only correctly folded and assembled proteins.