Biosynthesis and assembly of murein lipoprotein in Escherichia coli.

Abstract

The cell envelope of gram-negative enteric bacteria consists of three morphologically and functionally distinct layers: the inner cytoplasmic membrane, the murein or peptidoglycan layer, and the outer membrane at the external surface.’ The difference in the protein composition of the cytoplasmic and outer membranes is reflected both in the SDS polyacrylamide gel profiles and in the distinct enzyme localization.’ The outer membrane contains a number of major proteins, one of which is the murein lipoprotein discovered by Braun and his coworkers.’ The primary translation product of lipoprotein mRNA is a precursor protein, the prolipoprotein, which differs biochemically from the mature murein lipoprotein in several ways.‘ Consequently, a number of post-translational modifications of prolipoprotein must take place prior or subsequent to its translocation to the site of cellular localization, the outer membrane of the cell envelope.’,6 These modifications include (1) the proteolytic removal of the signal peptide containing 20 extra amino acids at the NH,-terminus of prolipoprotein; (2) the transfer of the glyceryl moiety from phosphatidylglycerol to the SH-group of cysteine in apolipoprotein’; (3) acylation of glycerylcysteine to form diglyceride-cysteine; (4) N-acylation of the NH,terminal cysteine; and (5 ) the joining of free form lipoprotein to the murein sacculus. The over-all pathway of the biosynthesis and assembly of murein lipoprotein is schematically summarized in FIGURE 1. Very little is known about the nature and the subcellular localizations of the lipid precursors and the putative enzymes participating in this series of biochemical reactions. Nor has it been established whether any of these modifications is related to the process of assembly of lipoprotein into the outer membrane and into the murein sacculus. Using both biochemical and genetic approaches, we have studied two aspects of lipoprotein assembly in some detail, i.e. the biosynthesis of the covalently linked lipid moiety in lipoprotein, and the necessity, if any, of the removal of the signal peptide in the assembly of lipoprotein.