Studies of the protein subunit of pH-inactivated Mengo virus variants: II. Physicochemical properties

Abstract

Incubation of three variants of Mengo virus at slightly acid pH's in the presence of chloride or bromide ions results in the dissociation of the viral capsid into protein subunits with the release of the intact viral genome. Analytical ultracentrifuge studies show that the protein subunits derived from all three variants are identical and are comprised of a single homogeneous molecular species which has a molecular weight of 407,000 ± 11,000 and an intrinsic sedimentation coefficient ( o 20,w) of 13.4 ± 0.13 S. The frictional ratio of the subunit estimated from the molecular weight and o 20,w is 1.45. On the basis of a hydrodynamically equivalent ellipsoidal model, the protein possesses a maximal axial ratio of eight. The hydrodynamic data, together with those reported recently (O'Callaghan et al., 1970b) suggest that eight to ten molecules of virus polypeptide I (VPI) and four to five molecules of virus polypeptide II (VPII) comprise the molecular anatomy of the subunit. The circular dichroic (cd) spectra of the subunits of all three variants exhibit comparable ellipticity bands at 208 and 225–228 nm with amplitudes of −4000° and −2500°, respectively, suggesting that the protein possesses an apparent α-helical content of approximately 5–10%. The virtual equivalence of the cd spectra for the three subunits suggests that the general orientation and internal arrangement of their polypeptide chains are similar if not identical. Comparison of the calculated and experimental cd profiles for the subunit reveals differences reflected in terms of an underlying positive cd contribution at 223 nm and a negative one at 209 nm. These differences are interpreted in terms of three possibilities: (1) interaction of the protein coat with the nucleic acid; (2) localized melting of the secondary structure of the viral RNA to allow folding into the tightly compact form found within the virion; (3) the absence of structural protein VPIII in the protein subunit.