Structure of a foreign peptide displayed on the surface of bacteriophage M13.

The use of filamentous bacteriophage M13 as a vehicle for display of foreign peptides and proteins provides a means for the construction of therapeutic, diagnostic and technological tools of broad utility. The usefulness of this technology is dependent on the ability of an inserted peptide to act as a ligand when fused to a structural protein. This, in turn, depends on the configuration in which the fused peptide is presented on the surface of the phage. X-ray diffraction from oriented fibers of three M13 strains with different sequences inserted near the amino terminus of the major coat protein (gp8) has been used to demonstrate that the inserts do not affect the helical symmetry of the phage particles. The structure of one insertion mutant (M13BOM2) was analyzed in detail. This strain contains the pentapeptide GQASG inserted between amino acids 4 and 5 of the major coat protein. Analysis of fiber diffraction from this strain was used to obtain its structure to 7 A resolution. Examination of the resulting electron density map indicated that the insert is presented in an extended conformation in a shallow groove between two alpha-helices on the surface of the virion. This arrangement is reminiscent of the presentation of peptides by major histocompatibility antigens. The extended conformation of the peptide provides substantial surface exposure and puts it in a favorable position to act as a ligand in a biochemical process. This form of presentation may contribute to the high immunogenicity observed for peptides inserted into the gene 8 product of M13. The length of the groove appears to correspond to the upper length limit observed when foreign peptides are fused to all copies of gp8.