Peptidase M48 subfamily B HtpX-like membrane-bound metallopeptidase
This family contains peptidase M48 subfamily B, also known as HtpX, which consists of proteins smaller than Ste24p, with homology restricted to the C-terminal half of Ste24p. HtpX, an integral membrane (IM) metallopeptidase, is widespread in bacteria and archaea, and plays a central role in protein quality control by preventing the accumulation of misfolded proteins in the membrane. Its expression is controlled by the Cpx stress response system, which senses abnormal membrane proteins. HtpX participates in the proteolytic quality control of these misfolded proteins by undergoing self-degradation and eliminating them by collaborating with FtsH, a membrane-bound and ATP-dependent protease. HtpX contains the zinc binding motif (HEXXH), has an FtsH-like topology, and is capable of introducing endoproteolytic cleavages into SecY (also an FtsH substrate). However, HtpX does not have an ATPase activity and will only act against cytoplasmic regions of a target membrane protein. Thus, HtpX and FtsH have overlapping and/or complementary functions, which are especially important at high temperature; in E. coli and Xylella fastidiosa, HtpX is heat-inducible, while in Streptococcus gordonii it is not. Mutation studies of HtpX-like M48 metalloprotease from Leptospira interrogans (LA4131) has been shown to result in altered expression of a subset of metal toxicity and stress response genes.
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