FIG. 3.

Effects of phospholipids on the activity of S. pneumoniae signal peptidase I. Prestreptokinase, the substrate, was purified from E. coli as described in the text and incubated with purified signal peptidase I in the presence of E. coli total lipid extract or different pure phospholipids. All samples were analyzed on an SDS–10% polyacrylamide gel and stained with Coomassie brilliant blue. Densitometer analysis was performed with a Personal Densitometer S1 and Image Quant 5.0 software from Molecular Dynamics. Lane 1, purified prestreptokinase only (10% of the mature streptokinase appearing in this lane was from purification), lanes 2 to 6, purified prestreptokinase incubated with purified signal peptidase I in the presence of E. coli lipid extract (lane 2), phosphatidylglycerol (lane 3), phosphatidylethenolamine (lane 4), cardiolipin (lane 5), and no phosphalipid (lane 6). Pre-Ska and mSka, prestreptokinase and mature streptokinase, respectively.