The adaptor protein Hop mediates the association of the molecular chaperones Hsp70 and Hsp90. The TPR1 domain of Hop specifically recognizes the C-terminal heptapeptide of Hsp70 while the TPR2A domain binds the C-terminal pentapeptide of Hsp90. Both sequences end with the motif EEVD. The crystal structures of the TPR-peptide complexes show the peptides in an extended conformation, spanning a groove in the TPR domains. Peptide binding is mediated by electrostatic interactions with the EEVD motif, with the C-terminal aspartate acting as a two-carboxylate anchor, and by hydrophobic interactions with residues upstream of EEVD. The hydrophobic contacts with the peptide are critical for specificity. These results explain how TPR domains participate in the ordered assembly of Hsp70-Hsp90 multichaperone complexes.

Bag (Bcl2-associated athanogene) domains occur in a class of cofactors of the eukaryotic chaperone 70-kilodalton heat shock protein (Hsp70) family. Binding of the Bag domain to the Hsp70 adenosine triphosphatase (ATPase) domain promotes adenosine 5'-triphosphate-dependent release of substrate from Hsp70 in vitro. In a 1.9 angstrom crystal structure of a complex with the ATPase of the 70-kilodalton heat shock cognate protein (Hsc70), the Bag domain forms a three-helix bundle, inducing a conformational switch in the ATPase that is incompatible with nucleotide binding. The same switch is observed in the bacterial Hsp70 homolog DnaK upon binding of the structurally unrelated nucleotide exchange factor GrpE. Thus, functional convergence has allowed proteins with different architectures to trigger a conserved conformational shift in Hsp70 that leads to nucleotide exchange.

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Prefoldin (GimC) is a hexameric molecular chaperone The archaeal Group II chaperonin (thermosome) is complex built from two related classes of subunits closely related to its eukaryotic homologue TRiC and present in all eukaryotes and archaea. Prefoldin (Gutsche et al., 1999). In contrast, Hsp70 proteins and interacts with nascent polypeptide chains and, in vitro, TF are generally missing from the archaeal kingdom can functionally substitute for the Hsp70 chaperone though some archaea have acquired Hsp70, presumably system in stabilizing non-native proteins for subseby lateral gene transfer (Gribaldo et al., 1999). However, quent folding in the central cavity of a chaperonin. all archaea contain a homologue of the recently de-Here, we present the crystal structure and characterscribed eukaryotic molecular chaperone prefoldin/GimC ization of the prefoldin hexamer from the archaeum (Geissler et al., 1998; Vainberg et al., 1998; Hansen et al., Methanobacterium thermoautotrophicum. Prefoldin 1999; Siegers et al., 1999), which is absent in bacteria. has the appearance of a jellyfish: its body consists of Archaeal prefoldin has been shown to have ATP-indea double ␤ barrel assembly with six long tentacle-like pendent chaperone properties similar to that of Hsp70 coiled coils protruding from it. The distal regions of in a folding pathway with a chaperonin in vitro (Leroux the coiled coils expose hydrophobic patches and are et al., 1999), and may perform Hsp70-like functions required for multivalent binding of nonnative proteins. in vivo. The structures and functions of Hsp70 and chaper-* To whom correspondence should be addressed (email: ismail@ tensions of their apical domains that are thought to memoarefi.com [I. M.],

The coordinated interaction of the Hsp70 and Hsp90 molecular chaperones is required for the folding and conformational regulation of a variety of signal transduction proteins and cell cycle regulators in the eukaryotic cytosol (reviewed in Refs.