
Full text loading...
Periplasmic Chaperones and Peptidyl-Prolyl Isomerases, Page 1 of 2
< Previous page Next page > /docserver/preview/fulltext/10.1128/9781555815806/9781555813987_Chap08-1.gif /docserver/preview/fulltext/10.1128/9781555815806/9781555813987_Chap08-2.gifAbstract:
The cellular functions of molecular chaperones include protecting newly synthesized polypeptides from misfolding or aggregation, and promoting disaggregation and refolding of stress-denaturated proteins. It was debated whether periplasmic chaperones might exist that could associate with newly exported polypeptides emerging at the translocation sites to assist their folding process, as cytoplasmic chaperones do with newly synthesized polypeptides emerging from the ribosomes. Many molecular chaperones, evolutionarily conserved from humans to bacteria, were initially identified because they are induced in response to stress conditions leading to the accumulation of intracellular nonnative proteins. Although not historically classified as molecular chaperones, enzymes that catalyze two potentially rate-limiting folding reactions, or folding catalysts, are abundant in the periplasm. Two different global searches for periplasmic chaperones suggested an important role of Skp in the folding pathway of outer membrane proteins. First, Skp was retained on an affinity column with Sepharose-bound OmpF, and second, by genetic screening, mutations in the skp gene were isolated based on their increased σEactivity. Peptidyl-prolyl isomerases (PPIases) are ubiquitous and highly conserved enzymes that catalyze prolyl cis-trans isomerization, an intrinsically slow and potentially rate-limiting reaction during in vitro protein refolding. In addition to protein disulfide isomerases and proteases, the periplasm contains at least five distinct protein-folding factors, including the nonessential molecular chaperone Skp, and four PPIases (Pipe, FepA, Sure, and Paid) that are individually, but not collectively, dispensable for viability in E. coli.