Heterogenous nucleation on small molecule crystals causes a monoclinic crystal form of bacteriorhodopsin (BR) in which trimers of this membrane protein pack differently than in native purple membranes. Analysis of single crystals by nano-electrospray ionization-mass spectrometry demonstrated a preservation of the purple membrane lipid composition in these BR crystals. The 2.9-Å x-ray structure shows a lipid-mediated stabilization of BR trimers where the glycolipid S-TGA-1 binds into the central compartment of BR trimers. The BR trimer͞lipid complex provides an example of local membrane thinning as the lipid head-group boundary of the central lipid patch is shifted by 5 Å toward the membrane center. Nonbiased electron density maps reveal structural differences to previously reported BR structures, especially for the cytosolic EF loop and the proton exit pathway. The terminal proton release complex now comprises an E194-E204 dyad as a diffuse proton buffer.

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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.],

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