Metabolic turnover of phosphorylation sites in simian virus 40 large T antigen

Four (groups of) phosphorylation sites exist in the large T antigen of simian virus 40, and they involve at least two serine and two threonine residues (Van Roy et al. J. Virol. 45:315-331, 1983). All the phosphorylation sites were found to be modified and again dephosphorylated at discrete rates, with phosphoserine residues having the highest turnover rate. The measured half-lives ranged between 3 h (for the carboxy-terminal phosphoserine site) and 5.5 h (for the amino-terminal phosphothreonine site). The influence of four temperature-sensitive A mutations on phosphorylation of large T antigen was also examined. At restrictive temperature, phosphorylation of the carboxy-terminal phosphoserine in mutated large T antigen was found to be particularly impaired. These data emphasize the physiological importance of the latter phosphorylation site.