Reconstitution of potato virus X in vitro. I. Properties of the dissociated protein structural subunits

The functional protein subunits of the X^HR strain of potato virus X (PVX) obtained when the virus was disrupted with lithium chloride sedimented in the analytical ultracentrifuge as a single boundary (s^o_20,w = 2.3 × 10^-13), migrated in polyacrylamide gels as a single band, eluted from Sephadex G-200 as a single peak (Stokes radius = 3.27 nm), and behaved during diffusion coefficient measurements as a monodisperse solute (D_20,w = 5.02 × 10^-7 cm² sec^-1). Amino acid and tryptic peptide analyses showed the molecular weight of these subunits to be 23,000. The dimensions of the functional subunits calculated from these data were consistent with the volume available for the subunit in the intact virus. Results of hydrogen-tritium exchange and circular dichroism studies showed that the subunits possessed appreciable α-helical structure under conditions (20°C, pH 6.10 to 6.50, low ionic strength) optimal for reconstitution of viruslike particles with viral RNA. The properties of functional protein subunits thus differ in several respects from those of denatured subunits prepared with pyridine (Shalla and Shepard, 1970). However, functional subunits were serologically identical to denatured subunits in Ouchterlony double diffusion tests against PVX D-protein antiserum.