Ribulose-1,5-bisphosphate carboxylase/oxygenase (rubisco) is responsible for almost all biological CO2 assimilation, but forms inhibited complexes with its substrate ribulose-1,5-bisphosphate (RuBP) and other sugar phosphates. The distantly related AAA+ proteins rubisco activase and CbbX remodel inhibited rubisco complexes to effect inhibitor release in plants and α-proteobacteria, respectively. Here we characterize a third class of rubisco activase in the chemolithoautotroph Acidithiobacillus ferrooxidans. Two sets of isoforms of CbbQ and CbbO form hetero-oligomers that function as specific activases for two structurally diverse rubisco forms. Mutational analysis supports a model wherein the AAA+ protein CbbQ functions as motor and CbbO is a substrate adaptor that binds rubisco via a von Willebrand factor A domain. Understanding the mechanisms employed by nature to overcome rubisco's shortcomings will increase our toolbox for engineering photosynthetic carbon dioxide fixation.

Hsp90 is an ATP-dependent molecular chaperone which assists the maturation of a large set of target proteins. Members of the highly conserved Hsp90 family are found from bacteria to higher eukaryotes, with homologues in different organelles. The core architecture of Hsp90 is defined by the N-terminal ATP binding domain followed by the middle domain and the C-terminal dimerization domain. A long, highly charged linker between the N-terminal domain and the middle domain is a feature characteristic for Hsp90s of eukaryotic organisms. We set out to clarify the function of this linker by studying the effects of deletions in this region in vivo and in vitro. Here we show that increasing deletions in the charged linker region lead to defects ranging from mild temperature sensitivity to a lethal phenotype. The lethal deletion variants investigated in this study still exhibit ATPase activity. Deletion of the charged linker ultimately causes a loss of Hsp90 regulation by co-chaperones, as the sensitivity for Aha1-mediated ATPase acceleration declines, and binding of p23/Sba1 is lost in non-viable deletion constructs. In vivo client assays additionally demonstrated that the deletion of the linker had a pronounced effect on the ability of Hsp90 to facilitate client activation. A partial reconstruction of the linker sequence showed that the supplementation by artificial sequences can rescue the functionality of Hsp90 and restore the conformational flexibility of the protein, required for the processing of client proteins. Hsp903 is an ATP-dependent molecular chaperone present in the cytosol of eubacteria and eucaryotes. It regulates the maturation and activation of numerous proteins involved in signal transduction, cell cycle control, hormone signaling, and transcription (1-3). Recent crystal structures of HtpG from Escherichia coli and Hsp90 from Saccharomyces cerevisiae show that the overall structural organization of the proteins is highly conserved (4 -6). Both prokaryotic and eukaryotic Hsp90 proteins consist of an N-terminal ATP binding domain, a middle domain involved in client protein binding, and a C-terminal dimerization domain. During the ATPase cycle, large conformational changes in the Hsp90 dimer lead to the transient dimerization of the N-terminal domains and their association with the middle domains (7-12). Despite the conservation of the basic molecular architecture and the ATPase mechanism, there are major differences between Hsp90 from prokaryotes and eukaryotes. The most striking difference is the emergence of a large set of co-chaperones in eucaryotes that bind to Hsp90 and seem to modulate and expand its properties (13). Furthermore, in contrast to procaryotes, Hsp90 is an essential protein in eucaryotes (14, 15) and it had been shown in S. cerevisiae that ATP hydrolysis by Hsp90 is required for sustaining its essential function (16 -18). Finally, eukaryotic Hsp90 contains additional structural elements compared with prokaryotic Hsp90; that is, a long charged linker between the N terminus and the middle...

The air-dried leaves of Tectona philippinensis, an endemic and endangered Philippine medicinal plant, afforded 5-hydroxy-3,7,4'-trimethoxyflavone (1), 5,4'-dihydroxy-3,7-dimethoxyflavone (2), squalene (3), a mixture of lupeol (4a) and beta-amyrin (4b), chlorophyllide a (5), and hydrocarbons. Antimicrobial tests on 1 and 2 indicated low antifungal activity against the fungi, Candida albicans and Trichophyton mentagrophytes. Compound 1 was also found to have low antibacterial activity against Escherichia coli and Pseudomonas aeruginosa.