Phosphorylation of SV40 large T antigen at threonine residues results in conversion to a lower apparent molecular weight form.

Simian virus 40 large T antigen (large T) was dephosphorylated with potato acid phosphatase at pH 5.5. Phosphate residues bound to serine were more susceptible to potato acid phosphatase than threonine-bound phosphate residues. Dephosphorylation of the threonine residues with potato acid phosphatase resulted in a reduced gel-electrophoretic mobility, while removal of the remaining phosphate groups had no effect on the mobility of large T. Pulse-chase experiments revealed a slower migrating form of newly synthesized large T which was converted by phosphorylation to a faster migrating form and had a half-life of approximately 1 h.