Export and activity of hybrid FhuA'-'Iut receptor proteins and of truncated FhuA' proteins of the outer membrane of Escherichia coli.

The FhuA protein in the outer membrane of Escherichia coli serves as a multifunctional receptor for the phages T5, T1, phi 80, for colicin M, for ferrichrome (Fe3+-siderophore) and for the structurally related antibiotic, albomycin. To determine structural domains required for these receptor functions and for export, a fusion protein between FhuA and Iut (receptor for Fe3+-aerobactin and cloacin DF13) was constructed. In the FhuA'-'Iut hybrid protein, 24 amino acids of FhuA were replaced by 19 amino acids, 18 of which were from Iut. The number of plaque forming units of phage T5 and T1 on cells expressing FhuA'-'Iut was nearly as high as on cells expressing plasmid-encoded wild-type FhuA. However, 10(7)-fold higher concentrations of phage phi 80 and 10(3) times more colicin M were required to obtain a zone of growth inhibition. Truncated FhuA' proteins in which the last 24 amino acids at the carboxy-terminus were replaced by 16 (FhuA'2) or 3 (FhuA'T) amino acids could hardly be detected on polyacrylamide electrophoretograms of outer membrane proteins, due to proteolytic degradation. Sensitivity of cells expressing FhuA'2 to phage T5 and T1 was reduced by several orders of magnitude and sensitivity to phage phi 80 and colicin M was totally abolished. In contrast, cells expressing FhuA'T were nearly as sensitive to pahge T5, T1, and phi 80 and to colicin M as cells containing FhuA'-'Iut. None of the constructs could grow on ferrichrome as sole iron source and none was sensitive to albomycin.(ABSTRACT TRUNCATED AT 250 WORDS)