Simian virus 40 large T antigen isoelectric focuses as multiple species with varying phosphate content.

Simian Virus 40 large T antigen (TAg), radiolabeled with [³2P]orthophosphate and with/without [³H]methionine, was found to isoelectric focus as four distinct species distributed in a highly reproducible pattern. These species have been found to differ in their ${}^{32}\text{P}{}^3\text{H}$ ratios indicating that individual monomers of large TAg are heterogeneous in their phosphate content and suggesting, further, that large TAg may be differentially phosphorylated in vivo. Also, by increasing the concentration of large TAg the more basic species can be displaced into the more acidic species suggesting that the different isoelectric focusing forms represent aggregates of large TAg with varying phosphate content.