Chaperone domains convert prolyl isomerases into generic catalysts of protein folding

Abstract

The cis/trans isomerization of peptide bonds before proline (prolyl bonds) is a rate-limiting step in many protein folding reactions, and it is used to switch between alternate functional states of folded proteins. Several prolyl isomerases of the FK506-binding protein family, such as trigger factor, SlyD, and FkpA, contain chaperone domains and are assumed to assist protein folding in vivo. The prolyl isomerase activity of FK506-binding proteins strongly depends on the nature of residue Xaa of the Xaa-Pro bond. We confirmed this in assays with a library of tetrapeptides in which position Xaa was occupied by all 20 aa. A high sequence specificity seems inconsistent with a generic function of prolyl isomerases in protein folding. Accordingly, we constructed a library of protein variants with all 20 aa at position Xaa before a rate-limiting cis proline and used it to investigate the performance of trigger factor and SlyD as catalysts of proline-limited folding. The efficiencies of both prolyl isomerases were higher than in the tetrapeptide assays, and, intriguingly, this high activity was almost independent of the nature of the residue before the proline. Apparently, the almost indiscriminate binding of the chaperone domain to the refolding protein chain overrides the inherently high sequence specificity of the prolyl isomerase site. The catalytic performance of these folding enzymes is thus determined by generic substrate recognition at the chaperone domain and efficient transfer to the active site in the prolyl isomerase domain.folding catalysis ͉ folding helpers ͉ folding mechanism ͉ SlyD ͉ trigger factor T he cis/trans isomerization of peptide bonds before proline (Xaa-Pro or prolyl bonds; Fig. 1A) is an intrinsically slow reaction that depends on the nature of the amino acid Xaa (1-3). It determines the rates of many protein folding reactions (4-6), is used as a molecular switch (7-16), and is catalyzed by prolyl isomerases (17-21). Most of the prolyl isomerases that assist in cellular protein folding contain catalytic domains that are homologous to human FKBP12 (FK-506 binding protein of 12 kDa; Fig. 1B) and chaperone domains, which interact transiently with non-native proteins (Fig. 1B). The trigger factor (22-25) and SlyD [product of the slyD (sensitive-to lysis) gene, a cytosolic Escherichia coli chaperone] (26-29) belong to this family of folding enzymes. The chaperone domain of SlyD (the ''insertin-flap'' or IF domain) shows a unique fold (30) and is structurally not related to other chaperones. The chaperone domain of trigger factor shows similarities with prefoldin (31)and SurA (32). FkpA (periplasmic FKBP of E. coli) (33-35) of prokaryotes and FKBP52 of eukaryotes (36) display similar combinations of prolyl isomerase and chaperone domains.FKBP-type prolyl isomerases are highly specific with regard to residue Xaa before the proline (37-39). An initial characterization of human FKBP12 with various proline-containing tetrapeptides and a protease-coupled assay (17) indicated that it catalyzes isomerizati...

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