The product of the F sex factor traT surface exclusion gene is a lipoprotein.

The product of the Escherichia coli sex factor F traT gene (TraTp), an outer membrane protein of M , = 25,000, is covalently modified in vivo by the addition of glycerol and fatty acids.Consistent with this result, and as would be expected for a bacterial lipoprotein, the novel amino acid glycerylcysteine can be detected in purified TraTp.Being a secreted protein, TraTp is made from a signal sequence containing precursor, and glycerol and fatty acids can be detected in both the precursor and mature (processed) species of TraTp.The peptide antibiotic globomycin inhibits the cleavage of the pro-TraTp signal sequence, but not the glycerol and fatty acid modification.Diglyceride modification of the Cys residue at the site of signal sequence cleavage probably precedes and is a prerequisite for processing of the TraTp signal sequence.Thus, TraTp appears to be a typical E. coli lipoprotein, having a pathway for modification and processing that is similar to that of Braun's lipoprotein (the major outer membrane lipoprotein).The truT gene of the Escherichia coli sex factor F codes for an outer membrane protein of molecular weight 25,000 with two important functions at the bacterial surface, namely resistance to killing by serum complement (1) and surface exclusion (2).Surface exclusion by the F plasmid causes its host cell to become a poor recipient in a mating with another donor cell carrying the same or a related plasmid.The truT gene product is made as a precursor molecule (3) with a hydrophobic signal sequence which is cleaved off during assembly into the outer membrane.'The mature and the precursor forms of the truT gene product (shortened as TraTp) can be separated by denaturing SDS'-polyacrylamide gel electrophoresis, and in such a separation the mature species (and not the precursor) has varying migration depending on the ionic strength of the separating gel.Such an unusual electrophoretic behavior suggested the possibility of covalent modification of the protein.Labeling experiments with glycerol and palmitic acid and the use of the peptide antibiotic globomycin (4) revealed that TraTp is a lipoprotein.