Subunit orientation in the filamentous virus Ff(fd, f1, M13)

The Ff filamentous viruses (fd, M13, f1) are important models for membrane protein assembly and are used extensively as cloning vectors and vehicles for peptide display. The thread-like virion (approximately 6 nm x 880 nm) comprises a single-stranded DNA genome sheathed by approximately 2700 copies of a 50-residue alpha-helical subunit (pVIII). The average inclination of pVIII subunits from the virion axis has been determined experimentally by polarized Raman microspectroscopy of oriented fd fibers, using the amide I Raman tensors obtained from a peptide single crystal. It is found that the average tilt angle, theta, between the pVIII alpha-helix and the virion axis falls within the range 13< theta <20 degrees.