Braun's lipoprotein is a major outer membrane protein in Escherichia coli and other gram-negative bacteria [I]. The biosynthesis of this protein has many unique features which have been extensively investigated [ 2 4 ] . The pathway for the biosynthesis of Braun's lipoprotein as it is currently understood is summarized in Figure 1 [5,6]. Biosynthesis of lipoprotein, as envisaged in this scheme, would require the specific products of seven genes. However, only one of these genes (the structural gene for the lipoprotein, Zpp) has thus far been identified and characterized [7-91. The genes encoding the modification and processing enzymes remain to be identified.
Braun's lipoprotein is one of many outer membrane proteins which are synthesized first as precursor proteins and subsequently processed to their mature forms. The processing of prolipoprotein, however, appears to follow a pathway distinct from those shared by many other outer membrane and periplasmic proteins. The first clue as to the unique mechanism of processing of prolipoprotein derived from the discovery of a novel cyclic peptide antibiotic, globomycin (Fig. 2) [lo]. The work of Arai and his co-workers on the mode of action of globomycin revealed that this antibiotic inhibits the processing of prolipoprotein but does not affect the processing of other major outer membrane proteins [ 111. This important observation strongly suggests that the signal peptidase for lipoprotein is unique and distinct from those required for the processing of other exported proteins.
Subsequent studies on the mechanism of action of globomycin have provided further insights into the biosynthesis of Braun's lipoprotein. The fact that prolipopro-