Chaperone network in the yeast cytosol: Hsp110 is revealed as an Hsp70 nucleotide exchange factor

The Hsp110 proteins, exclusively found in the eukaryotic cytosol, have significant sequence homology to the Hsp70 molecular chaperone superfamily. Despite this homology and the cellular abundance of these proteins, the precise functional role has remained undefined. Here, we present the intriguing finding that the yeast homologue, Sse1p, acts as an efficient nucleotide exchange factor (NEF) for both yeast cytosolic Hsp70s, Ssa1p and Ssb1p. The mechanism involves formation of a stable nucleotide-sensitive complex, but does not require ATP hydrolysis by Sse1p. The NEF activity of Sse1p stimulates in vitro Ssa1p-mediated refolding of thermally denatured luciferase, and appears to have an essential role in vivo. Overexpression of the only other described cytosolic NEF, Fes1p, can partially compensate for a lethal sse1,2Δ phenotype, however, the cells are sensitive to stress conditions. Furthermore, in the absence of Sse, the in vivo refolding of thermally denatured model proteins is affected. This is the first report of a nucleotide exchange activity for the Hsp110 class of proteins, and provides a key piece in the puzzle of the cellular chaperone network

Biochemistry and Genetics of Bacterial Bioluminescence

Bacterial light production involves enzymes-luciferase, fatty acid reductase, and flavin reductase-and substrates-reduced flavin mononucleotide and long-chain fatty aldehyde-that are specific to bioluminescence in bacteria. The bacterial genes coding for these enzymes, luxA and luxB for the subunits of luciferase; luxC, luxD, and luxE for the components of the fatty acid reductase; and luxG for flavin reductase, are found as an operon in light-emitting bacteria, with the gene order, luxCDABEG. Over 30 species of marine and terrestrial bacteria, which cluster phylogenetically in Aliivibrio, Photobacterium, and Vibrio (Vibrionaceae), Shewanella (Shewanellaceae), and Photorhabdus (Enterobacteriaceae), carry lux operon genes. The luminescence operons of some of these bacteria also contain genes involved in the synthesis of riboflavin, ribEBHA, and in some species, regulatory genes luxI and luxR are associated with the lux operon. In well-studied cases, lux genes are coordinately expressed in a population density-responsive, self-inducing manner called quorum sensing. The evolutionary origins and physiological function of bioluminescence in bacteria are not well understood but are thought to relate to utilization of oxygen as a substrate in the luminescence reaction