A POLYPEPTIDE BOUND BY THE CHAPERONIN GROEL IS LOCALIZED WITHIN A CENTRAL CAVITY

Chaperonins are oligomeric protein complexes that play an essential ro le in the cell, mediating ATP-dependent polypeptide chain folding in a variety of cellular compartments. They appear to bind early folding i ntermediates, preventing their aggregation; in the presence of MgATP a nd a cochaperonin, bound polypeptides are released in a stepwise manne r, associated with folding to the native state. Chaperonin complexes a ppear in the electron microscope as cylindrical structures, usually co mposed of two stacked rings, each containing, by negative staining, an electron dense central ''hole'' almost-equal-to 6.0 nm in diameter. W e sought to identify the site on the Escherichia coli chaperonin groEL , where the ''molten globule''-like intermediate of dihydrofolate redu ctase (DHFR) becomes bound, by examining in the scanning transmission electron microscope complexes formed between groEL and DHFR molecules bearing covalently crosslinked 1.4-nm gold clusters. In top views of t he groEL complexes, gold densities were observed in the central region ; in side views, the densities were seen at the end portions of the cy linders, corresponding to positions within the individual rings. In so me cases, two gold densities were observed in the same groEL complex. We conclude that folding intermediates are bound inside central caviti es within individual chaperonin rings. In this potentially sequestered location, folding intermediates with a compact conformation can be bo und at multiple sites by surrounding monomeric members of the ring; lo calization of folding within the cavity could also facilitate rebindin g of structures that initially fail to incorporate properly into the f olding protein.