The reassociation of the monomeric sheath protein, the product of gene 18, with the core-baseplate was investigated by analytical ultracentrifugation, light-scattering and electron microscopy.
The following conclusions are reached: (1) monomeric P18 molecules are in equilibrium with the extended tail sheath; (2) the association process is co-operative and the critical concentration of P18 is about 0.4 μm in the presence of 0.1 m-KCl in 1 mm-potassium phosphate buffer (pH 7.0 at 20 °C); (3) binding of P18 to the baseplate-core junction is the initial stop in extended sheath formation; (4) slow, irreversible polysheath formation competes with the assembly of extended sheath, but the latter is kinetically much more favored.
Model calculations on the isotherm of the sheath formation and on the length distribution strongly suggest a rate-limiting nucleation step, and a distinctly strong binding of the last annulus of the sheath to the core-baseplate.