Filamentous Bacteriophage Pf1 Has 27 Subunits in Its Axial Repeat

Abstract

Molecular model building is now being applied to analyze the structure of the filamentous phage coat protein (Marvin and Wachtel 1975Marvin and Wachtel 1976) by using information from low-resolution fiber diffraction and amino acid sequence without recourse to conventional methods of protein crystallography. This approach has important implications for the study of the structure of structural proteins that aggregate in their own purposeful way to build symmetric filaments, shells, or sheets rather than crystals suitable for high-resolution diffraction analysis.

The general picture of the Pf1 phage structure (Marvin et al. 1974a; Marvin and Wachtel 1975; Nakashima et al. 1975) is that the 46-residue protein subunit is largely α-helical; the subunits are interlocked in a regular, overlapping spiral arrangement to form a protein tube about 60 Å in diameter with a wall about 20 Å thick. This helical tube contains the two strands of the extended circular DNA molecule; the positively charged C-terminal end of the subunit is at the inside of the tube near the DNA and the acidic N terminus is at the outside surface.

Marvin and Wachtel (1976) have constructed a detailed molecular model of the Pf1 protein coat by interlocking α-helical subunits in a regular helical assembly. This model presumes that there is one subunit every 3.4 Å along the axis of the particle; i.e., 4.4 subunits per turn of the 15-Å pitch helix or 22 subunits in the 75-Å helix repeat period. (The repeat period varies slightly with changes in humidity, from about 72 Å for completely dry...