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Analysis of the protein composition of t.omato spott.ed wilt virus (TSWV), purified by an improved procedure, by polacrylamide gel electrophoresis, revealed t,hree major structural proteins (of MW 84,000, 50,000, and 29,OOOdj and a minor one of MX' 220,000d. The three major proteins constitute about 98?;, of the total viral protein and all three were shown to be glycoproteins.One of t.he major proteins (MW 29,000d) and the minor protein were shown t.o be associat.ed with subviral particles isolated bJ t,reatment of virus with the nonionic detergent. Nonidet P-40. Only traces of the other two proteins were detected in t.he subviral particles.Synthesis of virus-induced proteins in TSWV-infected t.obacco leaves was studied by labeling infected and healthy tissue with ["HI and [lJC]valine, respectively. The labeled tissues were then fractionated into crude subcellular fractions and protein patterns of healthy and infected tissues were compared by coelect,rophoresis on polyacrylamide gels. Only one virus-specific protein (of MW J9,OOOd) was detected in the virus-enriched fractions; t,his corresponded with t.he viral structural protein of MW 50,000d. 12

SUMMARYParticles of a stable isolate of tomato spotted wilt virus (TSWV) had a density of 1-147 g/ml in sucrose and contained four major structural proteins of mol. wt.: 1, 26 × 103; 2, 52× 103; 3, 56× 103; and 4, 78× 103 . The RNA which was not infectious consisted of four segments ofmol, wt.: 1, 3.6 x 106; 2, 2.8 × 106; 3, 2.0 × 106; and 4, 1.3 × 106. The ribonucleoprotein (RNP) of TSWV, prepared by treatment of purified virus with Nonidet P-40, could be separated into three components by sedimentation on sucrose density-gradients. All three components contained protein 1; the slowest sedimenting component also contained RNA species 1, 2 and 4, the second component contained RNA species 1 and 2 and the fastest sedimenting component contained all four RNA species. The three components were slightly infectious and infectivity was enhanced by pooling these components. The RNP, when observed under the electron microscope, appeared to consist of strands.Enveloped particles devoid of the outer layer of projections were obtained by treating TSWV with bromelain. These subviral particles retained about 1% of their original infectivity, had a density of 1.137 g/ml in sucrose and only contained protein 1. It was concluded that this protein forms the subunit of the nucleocapsid and that TSWV, unlike the majority of enveloped viruses, does not possess an internal membrane protein.