Abstract
Protein folding in the cellular environment involves an interplay between the intrinsic biophysical properties of a protein, in both its folded and unfolded states, and various accessory proteins that aid the process. Factors such as peptidyl prolyl isomerase, protein disulfide isomerase, thioredoxin, and SecB may interact with the unfolded forms of specific classes of proteins, while members of the hsp70/DnaK and hsp60/GroEL molecular chaperone families may play a more general role in folding. Secretion, proteolysis, and aggregation are other in vivo processes that depend greatly on the folding behavior of a given protein. Intrinsic folding rates, or even translation rates, of nascent proteins may be optimized by natural selection to ensure smooth coordination with all the cellular components required for a successful folding reaction.