Complex of simian virus 40 large tumor antigen and 48,000-dalton host tumor antigen.

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Abstract
SV-40 large tumor antigen (T Ag; a DNA-binding protein) can be separated by sucrose gradient sedimentation into a rapidly sedimenting, maximally phosphorylated fraction and a slowly sedimenting, less phosphorylated fraction. The 48,000 MW [mouse] host tumor antigen (48,000 HTA, also called nonviral T Ag) is preferentially complexed with the maximally phosphorylated T Ag. Pulse-labeled T Ag sediments as a 5-6S monomer; T Ag radiolabeled for progressively longer periods slowly increases in sedimentation coefficient to give a broad distribution between 5 S and greater than 28 S. Mutation in the viral A locus causes a decrease in T Ag phosphorylation and a marked decrease in 48,000 HTA binding, shifting the sedimentation coefficient of T Ag to the monomer value. The more highly phosphorylated T Ag also has the highest affinity for chromatin.