Messenger Ribonucleoproteins of Cells Infected by Simian Virus 40 Contain Large T‐Antigen

In monkey or mouse cells undergoing lytic or transforming infection with simian virus 40, about 10% of large tumor antigen (T-antigen) molecules were consistently present in the cytoplasm and 90% in the nucleus. The bulk of cytoplasmic large T-antigen cosedimented in linear sucrose gradients with polyribosomes (150-500 S) and with messenger ribonucleoproteins sedimenting within 20-80 S. As determined by centrifugation in discontinuous sucrose gradients, its apparent density (1.2-1.3 g/ml) corresponded to that of ribonucleoproteins. In contrast, the bulk of nuclear T-antigen sedimented between 5-20 S and its apparent density (1.1 g/ml) corresponded to that of free protein. Nuclear T-antigen added before cell fractionation did not bind to cytoplasmic constituents. After dissociation of purified polyribosomes with puromycin/KCl or EDTA, cytoplasmic large T-antigen cosedimented with the released messenger ribonucleoproteins containing poly(A)-rich messenger RNA. Upon hydrolysis of the RNA with RNase A, large T-antigen exhibited the sedimentation properties and density of free protein. The results suggest that cytoplasmic large T-antigen is associated with messenger ribonucleoproteins.