Description:
<jats:title>Abstract</jats:title>
<jats:p> The composition of the A-protein of tobacco mosaic virus (TMV) has been investigated by sedimentation velocity, light-scattering, spectroscopic methods, and thermodynamic calculations, at concentrations from 5 to 20 mg/ml, at temperatures from 7 to 26 °C , at various pH and buffer conditions. Above distinct critical concentrations and temperatures aggregates are formed which sediment near 8S, while the concentration of the smaller aggregates that sediment in the trailing boundary, near 4S, remains approximately constant. We identify the 4S protein with two-layer aggregates (Durham and Klug, J. Mol. Biol., 1972), with weight average molecular weight (M<jats:sub>w</jats:sub>) near 5 subunits, at the lower limit of polymerization. The 8S aggregates are best described by a series of three-layer aggregates, starting with a heptamer (Caspar, Adv. Protein Chem., 1963), but attaining M̅<jats:sub>w</jats:sub> corresponding to at least 12 subunits, at the upper limit. The 4S/8S equilibrium is not significantly changed by a change in pH, nor by the coexistence of higher aggregates (20 - 30S) with residual “A-protein”. Three-layer aggregates are more stable than two-layer aggregates, but significantly less stable than would be expected with strictly equivalent bonding; the third layer in the 8S protein disturbs the pairing between the two layers in the 4S protein, and the intersubunit interaction near tryptophan 52 seems to be involved. From the structure, 8S protein should tend to polymerize easier to helices than to disks, in accordance with earlier suggestions (Vogel et al., Eur. J. Biochem., 1977), and corroborated by studies on TMV-mutants.</jats:p>