Leader peptidase.

The Escherichia coli leader peptidase has been vital for unravelling problems in membrane assembly and protein export. The role of this essential peptidase is to remove amino-terminal leader peptides from exported proteins after they have crossed the plasma membrane. Strikingly, almost all periplasmic proteins, many outer membrane proteins, and a few inner membrane proteins are made with cleavable leader peptides that are removed by this peptidase. This enzyme of 323 amino acid residues spans the membrane twice, with its large carboxyl-terminal domain protruding into the periplasm. Recent discoveries show that its membrane orientation is controlled by positively charged residues that border (on the cytosolic side) the transmembrane segments. Cleavable pre-proteins must have small residues at -1 and a small or aliphatic residue at -3 (with respect to the cleavage site). Leader peptidase does not require a histidine or cysteine amino acid for catalysis. Interestingly, serine 90 and aspartic acid 153 are essential for catalysis and are also conserved in a mitochondrial leader peptidase, which is 30.7% homologous with the bacterial enzyme over a 101-residue stretch.