The smallest protein of Sendai virus: its candidate function of binding nucleocapsid to envelope

The smallest nonglycosylated protein, VP5 (MW, 35,000), of Sendai virus was investigated for its location and function. Virion-associated VP5 resisted digestion by trypsin, fungal semi-alkali protease, or chymotrypsin, suggesting that VP5 is not an external protein. A close association of VP5 with both envelope and nucleocapsid was first established by polypeptide analyses and electron microscopic examinations of fractions obtained by a rate zonal centrifugation of alkali-Tween 20-treated virions. Second, by CsCl equilibrium centrifugation of the same alkali-Tween 20-disrupted virions, nucleocapsids free of VP5 (rho = 1.323) and those associated with VP5 (rho = 1.297) were separated. The former had a width of 18 nm, but the latter, a broad width ranging from 18 to 40 nm (mean, 30 nm). Third, Triton X-100 treatment of the virions without KCl left VP5 in association with nucleocapsid, and, by electron microscopy, globular structures consisting of folded nucleocapsid were seen, while the same treatment with 1 M KCl completely removed VP5 from nucleocapsid and straightened nucleocapsids were observed. These three results suggest that VP5 bound to nucleocapsid is functioning to associate it to envelope and to sustain the spherical shape of the virion.