Regulation of Signal Peptidase by Phospholipids in Membrane: Characterization of Phospholipid Bilayer Incorporated Escherichia coli Signal Peptidase

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Abstract
Prokaryotic signal peptidases are membrane-bound enzymes. They cleave signal peptides from precursors of secretary proteins. To study the enzyme in its natural environment, which is phospholipid bilayers, we developed a method that allows us effectively to incorporate full-length Escherichia coli signal peptidase I into phospholipid vesicles. The membrane-bound signal peptidase showed high activity on a designed substrate. The autolysis site of the enzyme is separated from its catalytic site in vesicles by the lipid bilayer, resulting in a dramatic decrease of the autolysis rate. Phosphotidylethanolamine, which is the most abundant lipid in Escherichia coli inner membrane, is required to maintain activity of the membrane-incorporated signal peptidase. The maximal activity is achieved at about 55% phosphotidylethanolamine. Negatively charged lipids, which are also abundant in Escherichiacoli inner membrane, enhances the activity of the enzyme too. Its mechanism, however, cannot be fully explained by its ability to increase the affinity of the substrate to the membrane. A reaction mechanism was developed based on the observation that cleavage only takes place when the enzyme and the substrate are bound to the same vesicle. Accordingly, a kinetic analysis is presented to explain some of the unique features of phospholipid vesicles incorporated signal peptidase, including the effect of lipid concentration and substrate-vesicle interaction.