The enzyme peptidyl-prolyl cis-trans isomerase (PPIase) was recently discovered in mammalian tissues and purified from porcine kidney. It catalyses the slow cis-trans isomerization of proline peptide (Xaa-Pro) bonds in oligopeptides and accelerates slow, rate-limiting steps in the folding of several proteins. Here, we report the N-terminal sequence of PPIase together with further chemical and enzymatic properties. The results indicate that this enzyme is probably identical to cyclophilin, a recently discovered mammalian protein which binds tightly to cyclosporin A (CsA). Cyclophilin is thought to be linked to the immunosuppressive action of CsA. The first 38 amino-acid residues of porcine PPIase and of bovine cyclophilin are identical and the two proteins both have a relative molecular mass of about 17,000 (ref. 7). The catalysis of prolyl isomerization in oligopeptides and of protein folding by PPIase are strongly inhibited in the presence of low levels of CsA. The activities of both PPIase and cyclophilin depend on a single sulphydryl group. At present it is unknown whether the inhibition of prolyl isomerase activity is related with the immunosuppressive action of CsA.

Rates of protein folding reactions vary considerably. Some denatured proteins regain the native conformation within milliseconds or seconds, whereas others refold very slowly in the time range of minutes or hours. Varying folding rates are observed not only for different proteins, but can also be detected for single polypeptide species. This originates from the co-existence of fast- and slow-folding forms of the unfolded protein, which regain the native state with different rates. The proline hypothesis provides a plausible explanation for this heterogeneity. It assumes that the slow-folding molecules possess non-native isomers of peptide bonds between proline and another residue, and that crucial steps in the refolding of the slow-folding molecules are limited in rate by the slow reisomerization of such incorrect proline peptide bonds. Recently the enzyme peptidyl-prolyl cis-trans isomerase (PPIase) was discovered and purified from pig kidney. It catalyses efficiently the cis in equilibrium trans isomerization of proline imidic peptide bonds in oligopeptides. Here we show that it also catalyses slow steps in the refolding of a number of proteins of which fast- and slow-folding species have been observed and where it was suggested that proline isomerization was involved in slow refolding. The efficiency of catalysis depends on the accessibility for the isomerase of the particular proline peptide bonds in the refolding protein chain.

The molecular chaperone GroE facilitates correct protein folding in vivo and in vitro. The mode of action of GroE was investigated by using refolding of citrate synthase as a model system. In vitro denaturation of this dimeric protein is almost irreversible, since the refolding polypeptide chains aggregate rapidly, as shown directly by a strong, concentration-dependent increase in light scattering. The yields of reactivated citrate synthase were strongly increased upon addition of GroE and MgATP. GroE inhibits aggregation reactions that compete with correct protein folding, as indicated by specific suppression of light scattering. GroEL rapidly forms a complex with unfolded or partially folded citrate synthase molecules. In this complex the refolding protein is protected from aggregation. Addition of GroES and ATP hydrolysis is required to release the polypeptide chain bound to GroEL and to allow further folding to its final, active state.

The Escherichia coli periplasmic peptidyl-prolyl isomerase (PPIase) SurA is involved in the maturation of outer membrane porins. SurA consists of a substantial N-terminal region, two iterative parvulin-like domains and a C-terminal tail. Here we show that a variant of SurA lacking both parvulin-like domains exhibits a PPIase-independent chaperone-like activity in vitro and almost completely complements the in vivo function of intact SurA. SurA interacts preferentially (>50-fold) with in vitro synthesized porins over other similarly sized proteins, leading us to suggest that the chaperone-like function of SurA preferentially facilitates maturation of outer membrane proteins.

Here we used the cold-shock protein CspB from Bacillus subtilis to study protein folding at an elementary level. The thermodynamic stability of this small five-stranded beta-barrel protein is low, but unfolding and refolding are extremely rapid reactions. In 0.6 M urea the time constant of refolding is about 1.5 ms, and at the transition midpoint (4 M urea) the folded and unfolded forms equilibrate in less than 100 ms. Both the equilibrium unfolding transition and the folding kinetics are perfectly described by a N<-->U two-state model. The validity of this model was confirmed by several kinetic tests. Folding intermediates could neither be detected at equilibrium nor in the folding kinetics. We suggest that the extremely rapid folding of CspB and the absence of folding intermediates are related phenomena.

The cold shock protein CspB from Bacillus subtilis is only marginally stable, but it folds extremely fast in a simple N reversible U two-state reaction. The corresponding cold shock proteins from the thermophile Bacillus caldolyticus and the hyperthermophile Thermotoga maritima show strongly increased conformational stabilities, but unchanged very fast two-state refolding kinetics. The absence of intermediates in the folding of B. subtilis CspB is thus not a corollary of its low stability. Rather, two-state folding and an unusually native-like activated state of folding seem to be inherent properties of these small all-beta proteins. There is no link between stability and folding rate, and numerous sequence positions exist which can be varied to modulate the stability without affecting the rate and mechanism of folding.

In the past few years, however, an increasing number of proteins have been discovered, which were proposed to be involved in the maturation and folding of nascent protein chains in the cell (Pelham, 1986(Pelham, , 1988 Rothman & Kornberg, 1986;Freedman, 1987). These proteins are found in the cytoplasm as well as in the endoplasmic reticulum (ER),1 the mitochondria, or the chloroplasts. They are thought to be involved in functions such as catalyzing protein folding, keeping protein chains in a nonnative state, or "guiding" proteins to their cellular association partners.When recombinant proteins are overexpressed in foreign hosts, the polypeptide chains frequently do not fold correctly, but they are deposited in the cell in an insoluble, nonnative form (Marston, 1986), which could mean that the correct "folding helpers" are missing. We review here current concepts for the mechanism of in vitro protein folding and evaluate their significance for folding processes in vivo and the role that could be ascribed to cellular components in this process.Physicochemical Data on the in Vitro Folding and Stability of Proteins (1) Thermodynamic Stability of Proteins. Folded proteins are usually stable in a thermodynamic sense at ambient temperature and at neutral pH; however, the difference in Gibbs free energy between the native and the unfolded state is generally small. Frequently, values in the range of-40 kJ/mol are found. Consequently, small shifts in the solvent conditions, f Writing of this paper was supported by a grant from the Volkswagenstiftung.

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