Mutations in the N-Terminus of the Major Coat Protein (pVIII, gp8) of Filamentous Bacteriophage Affect Infectivity

Abstract

Filamentous bacteriophages are nonlytic, male-specific bacteriophages which infect Escherichia coli carrying an F-episome. The molecular mechanism of infection remains elusive, including the role of the major coat protein pVIII. In order to evaluate the contributions of major coat protein pVIII in the process of infection, two phage display libraries were generated. One library consisted of random amino acids at positions 2, 4, 5, 8, 11 and 12 of the N-terminus of major coat protein pVIII. The second library was generated by randomizing these positions as well as position 1. All these residues were previously shown to be exposed at the surface of the virions by being accessible to ligands. The infectivity of randomly selected mutant phages was analyzed. The present results demonstrate that phages modified at these positions can be correctly assembled and secreted into the exoplasm, although the efficiency was slightly lower than that of wild-type phage. Their infectivity varied greatly, and a general structural pattern underlying infectivity did not emerge. However, residual differences were observed between infectious and defective phage; in general, uncharged polar amino acids present at positions 5 and 11 of the N-terminus of pVIII reduced phage infectivity, whereas polar residues at position 8 facilitated infection. The first position of pVIII is remarkably critical for infection; when this alanine was substituted with other residues, most of the phages lost their infectivity. These results shed new light on the true complexity of random peptide pVIII phage display libraries.