Prolyl isomerases catalyze the cis/trans isomerization of peptide bonds preceding proline. Previously, we had determined the specificity toward the residue before the proline for cyclophilin-, FKBP-, and parvulin-type prolyl isomerases by using proline-containing oligopeptides and refolding proteins as model substrates. Here, we report the specificities of members of these three prolyl isomerase families for the residue following the proline, again in short peptide and in refolding protein chains. Human cyclophilin 18 and parvulin 10 from Escherichia coli show high activity, but low specificity, with respect to the residue following the proline. Human FKBP12 prefers hydrophobic residues at this position in the peptide assays and shows a very low activity in the protein folding assays. This activity was strongly improved, and the sequence specificity was virtually eliminated after the insertion of a chaperone domain into the prolyl isomerase domain of human FKBP12.
Folding enzymes often use distinct domains for the interaction with a folding protein chain and for the catalysis of intrinsically slow reactions such as prolyl cis/trans isomerization. Here, we investigated the refolding reaction of ribonuclease T1 in the presence of the prolyl isomerase SlyD from Escherichia coli to examine how this enzyme catalyzes the folding of molecules with an incorrect trans proline isomer and how it modulates the conformational folding of the molecules with the correct cis proline. The kinetic analysis suggests that prolyl cis → trans isomerization in the SlyD-bound state shows a rate near 100 s(-1) and is thus more than 10(4)-fold accelerated, relative to the uncatalyzed reaction. As a consequence of its fast binding and efficient catalysis, SlyD retards the conformational folding of the protein molecules with the correct cis isomer, because it promotes the formation of the species with the incorrect trans isomer. In the presence of ≥1 μM SlyD, protein molecules with cis and trans prolyl isomers refold with identical rates, because SlyD-catalyzed cis/trans equilibration is faster than conformational folding. The cis or trans state of a particular proline is thus no longer a determinant for the rate of folding.
Parvulins are small prolyl isomerases and serve as catalytic domains of folding enzymes. SurA (survival protein A) from the periplasm of Escherichia coli consists of an inactive (Par1) and an active (Par2) parvulin domain as well as a chaperone domain. In the absence of the chaperone domain, the folding activity of Par2 is virtually abolished. We created a chimeric protein by inserting the chaperone domain of SlyD, an unrelated folding enzyme from the FKBP family, into a loop of the isolated Par2 domain of SurA. This increased its folding activity 450-fold to a value higher than the activity of SurA, in which Par2 is linked with its natural chaperone domain. In the presence of both the natural and the foreign chaperone domain, the folding activity of Par2 was 1500-fold increased. Related and unrelated chaperone domains thus are similarly efficient in enhancing the folding activity of the prolyl isomerase Par2. A sequence analysis of various chaperone domains suggests that clusters of exposed methionine residues in mobile chain regions might be important for a generic interaction with unfolded protein chains. This binding is highly dynamic to allow frequent transfer of folding protein chains between chaperone and catalytic domains.
SlpA (SlyD-like protein A) comprises two domains, a FK506 binding domain (FKBP fold) of moderate prolyl cis/trans-isomerase activity and an inserted in flap (IF) domain that hosts its chaperone activity. Here we present the nuclear magnetic resonance (NMR) solution structure of apo Escherichia coli SlpA determined by NMR that mirrors the structural properties seen for various SlyD homologues. Crucial structural differences in side-chain orientation arise for F37, which points directly into the hydrophobic core of the active site. It forms a prominent aromatic stacking with F15, one of the key residues for PPIase activity, thus giving a possible explanation for the inherently low PPIase activity of SlpA. The IF domain reveals the highest stability within the FKBP-IF protein family, most likely arising from an aromatic cluster formed by four phenylalanine residues. Both the thermodynamic stability and the PPIase and chaperone activity let us speculate that SlpA is a backup system for homologous bacterial systems under unfavorable conditions.
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