Biochemical characterization of signal peptidase I from gram-positive Streptococcus pneumoniae

Bacterial signal peptidase I is responsible for proteolytic processing of t he precursors of secreted proteins. The enzymes from gram-negative and -pos itive bacteria are different in structure and specificity. In this study, w e have cloned, expressed, and purified the signal peptidase I of gram-posit ive Streptococcus pneumoniae. The precursor of streptokinase, an extracellu lar protein produced in pathogenic streptococci, was identified as a substr ate of S, pneumoniae signal peptidase I. Phospholipids were found to stimul ate the enzymatic activity. Mutagenetic analysis demonstrated that residues serine 38 and lysine 76 of S. pneumoniae signal peptidase I are critical f or enzyme activity and involved in the active site to form a serine-lysine catalytic dyad, which is similar to LexA-like proteases and Escherichia col i signal peptidase I. Similar to LexA-like proteases, S. pneumoniae signal peptidase I catalyzes an intermolecular self-cleavage in vitro, and an inte rnal cleavage site has been identified between glycine 36 and histidine 37, Sequence analysis revealed that the signal peptidase I and LexA-like prote ases show sequence homology around the active sites and some common propert ies around the self-cleavage sites. All these data suggest that signal pept idase I and LexA-like proteases are closely related and belong to a novel c lass of serine proteases.