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HslVU
The heat shock proteins hslV and hslU (also known as clpQ and clpY respectively〔Ramachandran R, Hartmann C, Song HK, Huber R, Bochtler M. (2002). Functional interactions of HslV (ClpQ) with the ATPase HslU (ClpY). ''Proc Natl Acad Sci USA'' 99(11):7396-401.〕) are expressed in many bacteria such as ''E. coli'' in response to cell stress. The hslV protein is a protease and the hslU protein is an ATPase; the two form a symmetric assembly of four stacked rings, consisting of an hslV dodecamer bound to an hslU hexamer, with a central pore in which the protease and ATPase active sites reside. The hslV protein degrades unneeded or damaged proteins only when in complex with the hslU protein in the ATP-bound state. The complex is thought to resemble the hypothetical ancestor of the proteasome, a large protein complex specialized for regulated degradation of unneeded proteins in eukaryotes, many archaea, and a few bacteria.〔Gille C, Goedel A, Schloetelburg C, Preißner R, Kloetzell PM, Gobel UB, Frommell C. (2003). A Comprehensive View on Proteasomal Sequences: Implications for the Evolution of the Proteasome. ''J Mol Biol'' 326: 1437–1448.〕
==Genetics==
Both proteins are encoded on the same operon within the bacterial genome. Unlike many eukaryotic proteasomes, which have several different peptide substrate specificities, hslV has a specificity similar to that of chymotrypsin; hence it is inhibited by proteasome inhibitors that specifically target the chymotrypsin site in eukaryotic proteasomes.〔Rohrwild M, Coux O, Huang HC, R P Moerschell RP, Yoo SJ, Seol JH, Chung CH, Goldberg AL. (1996). HslV-HslU: A novel ATP-dependent protease complex in Escherichia coli related to the eukaryotic proteasome. ''Proc Natl Acad Sci USA'' 93(12): 5808–5813〕 Although the HslVU complex is stable on its own, some evidence suggests that the complex is formed ''in vivo'' in a substrate-induced manner due to a conformational change in the hslU-substrate complex that promotes hslV binding.〔Azim MK, Goehring W, Song HK, Ramachandran R, Bochtler M, Goettig P. (2005). Characterization of the HslU chaperone affinity for HslV protease. ''Protein Sci'' 14(5):1357-62.〕
HslV and hslU genes have also been identified in some eukaryotes, although these also require the constitutively expressed proteasome for survival. These eukaryotic HslVU complexes assemble to apparently functional units, suggesting that these eukaryotes have both functional proteasomes and functional hslVU systems.〔Ruiz-Gonzalez MX, Marin I. (2006). Proteasome-related HslU and HslV genes typical of eubacteria are widespread in eukaryotes. ''J Mol Evol'' 63(4):504-12.〕
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