A family of membrane-embedded metalloproteases involved in regulated proteolysis of membrane-associated transcription factors

We present evidence that the sporulation protein SpoIVFB of Bacillus subtil is is a member of a newly recognized family of metalloproteases that have c atalytic centers adjacent to or within the membrane. SpoIVFB is required fo r converting the membrane-associated precursor protein, pro-sigma(K), to th e mature and active transcription factor sigma(K) by proteolytic removal of an N-terminal extension of 20 amino acids. SpoIVFB and other family member s share the conserved sequence HEXXH, a hallmark of metalloproteases, as we ll as a second conserved motif NPDG, which is unique to the family. Both mo tifs, which are expected to form the catalytic center of the protease, over lap hydrophobic segments that are predicted to be separate transmembrane do mains. The only other characterized member of this family of membrane-embed ded metalloproteases is the mammalian Site-2 protease (S2P), which is requi red for the intramembrane cleavage of the eukaryotic transcription factor s terol regulatory element binding protein (SREBP). We report that amino acid substitutions in the two conserved motifs of SpoIVFB impair pro-sigma(K) p rocessing and sigma(K)-directed gene expression during sporulation. These r esults and those from a similar analysis of S2P support the interpretation that both proteins are founding members of a family of metalloproteases inv olved in the activation of membrane-associated transcription factors. Thus, the pathways that govern the activation of the prokaryotic transcription f actor pro-sigma(K) and the mammalian transcription factor SREBP not only ar e analogous but also use processing enzymes with strikingly homologous feat ures.