The potential active site of the lipoprotein-specific (type II) signal peptidase of Bacillus subtilis

Type II signal peptidases (SPase II) remove signal peptides from lipid-modi fied preproteins of eubacteria. As the catalytic mechanism employed by type II SPases was not known, the present studies were aimed at the identificat ion of their potential active site residues. Comparison of the deduced amin o acid sequences of 19 known type II SPases revealed the presence of five c onserved domains. The importance of the 15 best conserved residues in these domains was investigated using the type II SPase of Bacillus subtilis, whi ch, unlike SPase II of Escherichia coli, is not essential for viability. Th e results showed that only six residues are important for SPase II activity . These are Asp-14, Asn-99, Asp-102, Asn-126, Ala-128, and Asp-129, Only As p-14 was required for stability of SPase II, indicating that the other five residues are required for catalysis, the active site geometry, or the spec ific recognition of lipid-modified preproteins. As Asp-102 and Asp-129 are the only residues invoked in the known catalytic mechanisms of proteases, m e hypothesize that these two residues are directly involved in SPase II-med iated catalysis, This implies that type II SPases belong to a novel family of aspartic proteases.