Peptidase M48 subfamily B HtpX-like membrane-bound metallopeptidase
This HtpX family of peptidase M48 subfamily B includes uncharacterized HtpX homologs and consists of proteins smaller than Ste24p, with homology restricted to the C-terminal half of Ste24p. HtpX 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 collaborating with FtsH, a membrane-bound and ATP-dependent protease, to eliminate them. HtpX, a zinc metalloprotease with an active site 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.