Signal peptidases in prokaryotes and eukaryotes--a new protease family

Signal peptidases remove targeting peptides from pre-proteins and play central roles in the secretory pathway, as well as in the delivery of proteins to the mitochondrial intermembrane space and to the lumen of thylakoids. The catalytic mechanism of pre-protein cleavage has long been an enigma, but recent data from site-directed mutagenesis and sequence alignment studies suggest that signal peptidases may constitute a new type of serine protease, mechanistically related to the beta-lactamases.