Orientation of Tryptophan-26 in Coat Protein Subunits of the Filamentous Virus Ff by Polarized Raman Microspectroscopy

The Ff filamentous virus, which includes the closely related strains fd, f1, and M13, serves as a model for membrane protein assembly and is employed extensively as a cloning vector and vehicle for peptide display. The threadlike virion (≈6 × 880 nm) comprises a single-stranded DNA genome sheathed by ≈2700 copies of a 50-residue α-helical subunit, the product of viral gene VIII. The pVIII subunit contains a single tryptophan residue (tryptophan-26) which is essential for assembly. We have employed polarized Raman microspectroscopy to determine the orientation of tryptophan-26 in pVIII subunits of oriented fd fibers. The present application is based upon the transfer of tryptophan Raman tensors from a recent study of N-acetyl-l-tryptophan single crystals [Tsuboi et al. (1996) J. Mol. Struct. 379, 43−50]. The polarized Raman spectra of fd indicate that the plane of the indole ring in each pVIII subunit is close to parallel to the virion axis. In this orientation, the line connecting indole ring atoms N1 and C2 is nearly perpendicular to the virion axis, while the indole pseudo-2-fold axis (a line connecting atom C2 to the midpoint of the C5−C6 bond) is approximately 36° from the virion axis. We have used the present results in combination with preferred tryptophan side-chain torsions [χ1 (C3−Cβ−Cα−N) and χ2,1 (C2−C3−Cβ−Cα)] in other proteins and a previously determined experimental value of χ2,1 in fd [Aubrey, K. L., & Thomas, G. J., Jr. (1991) Biophys. J. 60, 1337−1349] to propose a detailed molecular model for the orientation of the tryptophan-26 side chain in the native virus.