Identification of an outer membrane protein of Escherichia coli, with a role in the coordination of deoxyribonucleic acid replication and cell elongation

Abstract

Protein G of molecular weight 15,000 is the 4th commonest protein in the outer membrane of E. coli B/r. From experiments described here on the relationship of protein G production to cell elongation and septation, the hypothesis is proposed that protein G is a structural protein of cell elongation. Furthermore, a surplus of protein G is produced when deoxyribonucleic acid (DNA) synthesis is arrested and septation is thereby prevented. Thus protein G may be an important coordination protein in E. coli for integration of DNA synthesis, cell envelope elongation, and septation. Inhibition of normal cell elongation in a rod configuration in E. coli B/r by the novel amidinopenicillanic acid FL1060 was accompanied by changes in the rate of appearance of protein G and several other outer membrane proteins. The rate of appearance of protein G decreased some 70% within 60 min, in parallel with termination of rounds of normal cell elongation. Filament inducing concentrations of nalidixic acid increased dramatically the rate of appearance of protein G. After 30 min a plateau level some 250% higher than the control value was reached. Similar kinetics were observed in parallel with filament formation induced by incubation of a dnaB mutant of E. coli at the nonpermissive temperature. No change in the rate of appearance of protein G was observed during cephalexin or benzylpenicillin induced filament formation, indicating that increased protein G production was not a secondary consequence of filamentation. Cells treated with FL1060 lost their ability to be induced for protein G formation, with nalidixic acid, in parallel with their loss of ability to initiate rounds of normal cell elongation. A pulse chase experiment demonstrated that the protein G appearing in the outer membrane as a consequence of inhibition of DNA synthesis was the result of de novo synthesis rather than of interconversion from previously synthetized protein species. A preliminary characterization of protein G revealed several similarities with the well characterized lipoprotein of the outer membrane of E. coli. A comparison of the incorporation of several 14C labeled amino acids into protein G and the lipoprotein revealed substantial differences, however, perhaps ruling out a simple relationship between these 2 proteins.