Partial purification, structure and infectivity of complete maize rough dwarf virus particles

Abstract

Maize rough dwarf virus, a reolike virus, was purified from roots of maize by clarification with the fluorocarbon 1,1,2-trifluoro-1,2,2-trichloroethane (Freon 113) or with carbon tetrachloride followed by sucrose density gradient centrifugation. The product consisted mostly of complete particles, but some subviral particles were present. The virus was spherical, varying from 63 to 70 nm in diameter under different staining conditions; it had a double capsid and typical reovirus structure with, probably, 92 morphological units in the outer capsid. In addition, each particle possessed 12 projecting spikes (A spikes) each some 11 nm long, one at each 5-fold symmetry axis. Various physical or chemical treatments stripped off the A spikes and part of the outer capsid to give a spherical 50–57 nm subviral particle possessing 12 previously hidden spikes (B spikes) each about 8 nm long, coaxial with the A spikes. Each B spike was implanted on a differentiated part of the inner capsid here called a baseplate. Treatment of whole virus or subviral particles with aqueous neutral potassium phosphotungstate produced smooth subviral particles without B spikes, and longer treatment with phosphotungstate or brief treatment with n-butanol also removed the nucleic acid to give ghosts exhibiting internal structures. Detached B spikes were composed of 5 morphological subunits surrounding a central hole. Infectivity of preparations was tested by injecting them into adult females of the planthopper vector Laodelphax striatellus, which were later screened for production duction of virus symptoms on maize and barley. Crude and purified preparations containing the large virus particles were infectious; Freon-treated preparations possessed the highest infectivity. Subviral preparations made by chloroform treatment were not or very slightly infectious and smooth subviral particles made by treatment with phosphotungstate were not infectious.