Structure and assembly of filamentous bacterial viruses

Filamentous bacterial viruses are flexible nucleoprotein rods, about 6 nm in diameter by 1000-2000 nm in length (depending on the virus strain). A protein shell encloses a central core of single-stranded circular DNA. The coat protein subunits forming the shell are largely alpha-helix, elongated in an axial direction, and also sloping radially, so as to overlap each other and give an arrangement of subunits reminiscent of scales on a fish. This arrangement of alpha-helices is rather like some models of myosin filaments. An early step in assembly of the virion is the formation of a complex between the viral DNA and an intracellular packaging protein that is not found in completed virions. Newly synthesized coat protein becomes associated with the plasma membrane of the cell. During the final steps of assembly, the packaging protein is displaced from the DNA and replaced by coat protein as the virion passes out through the plasma membrane of the host cell.