Cellular folding prolyl isomerization

The conformation of Xaa-Pro peptide bonds in the newly synthesized polypeptide chains prior to cellular folding is not known. The product of protein biosynthesis could be a uniform chain with all peptide bonds in the trans conformation. If this chain starts to fold immediately, then the trom-prolines would be in the correct conformation already, the cis-prolines would be in the incorrect isomeric state, and their trans — cis isomerization would be involved in the folding of all molecules. Alternatively, if there is sufficient time available for the Xaa-Pro bonds of the nascent chains to reach a cis/trans equilibrium (e.g., when folding is transiently arrested by binding to other proteins, such as heat-shock protein (HSP70), then the distribution of prolyl cis and trans isomers prior to cellular folding could be similar to the distribution found in the unfolded protein in vitro. Such a case was encountered in the maturation... 

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