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 f d, fl, and M13, serves as a model for membrane protein assembly and is employed extensively as a cloning vector and vehicle for peptide disp lay. The threadlike virion (approximate to 6 x 880 nm) comprises a sin gle-stranded DNA genome sheathed by approximate to 2700 copies of a 50 -residue alpha-helical subunit, the product of viral gene VIII. The pV III subunit contains a single tryptophan residue (tryptophan-26) which is essential for assembly. We have employed polarized Raman microspec troscopy to determine the orientation of tryptophan-26 in pVIII subuni ts of oriented fd fibers. The present application is based upon the tr ansfer of tryptophan Raman tensors from a recent study of N-acetyl-L-t ryptophan single crystals [Tsuboi et al. (1996) J. Mol. Struct. 379, 4 3-50]. The polarized Raman spectra of fd indicate that the plane of th e indole ring in each pVIII subunit is close to parallel to the virion axis. In this orientation, the line connecting indole ring atoms N1 a nd C2 is nearly perpendicular to the virion axis, while the indole pse udo-2-fold axis (a line connecting atom C2 to the midpoint of the C5-C 6 bond) is approximately 36 degrees from the virion axis. We have used the present results in combination with preferred tryptophan side-cha in torsions [chi(1) (C3-C beta-C alpha-N) and chi(2,1) (C2-C3-C beta-C alpha)] in other proteins and a previously determined experimental va lue of chi(2,1) in fd [Aubrey, K. L., & Thomas, G. J., Jr. (1991) Biop hys. J. 60, 1337-1349] to propose a detailed molecular model for the o rientation of the tryptophan-26 side chain in the native virus.