Post-translational modification and processing of outer membrane prolipoproteins in Escherichia coli.

This mini review is primarily concerned with the biosynthesis of the major outer membrane lipoprotein of Escherichia coli. The lipoprotein is composed of fifty-eight amino acid residues, one glyceride residue and one acyl residue being at the amino terminal cysteine residue. About one-third of the lipoprotein is covalently bound to the underlying peptidoglycan layer and plays an important role in the assembly of the outer membrane on the peptidoglycan layer. The lipoprotein is first synthesized on ribosomes as a precursor form having twenty extra amino acid residues (signal peptide) at the amino terminus. During secretion through the cytoplasmic membrane, the modification at the cysteine residue that is to be the amino terminus of the mature lipoprotein and cleavage of the signal peptide take place successively. These events are then followed by N-acetylation of the terminal cysteine residue, translocation to the outer membrane, and covalent binding to the peptidoglycan layer of the lipoprotein. Digestion of the cleaved signal peptide also takes place upon cleavage of the signal peptide. In this review these chemical and topographical events are discussed step by step especially in relation to the process of protein secretion across the cytoplasmic membrane.