Two classes of phosphotyrosine-containing proteins detected by labeling with 32P and 125I in HeLa cells.

There are two classes of proteins that can be phosphorylated on tyrosine in HeLa cells. One class can be detected by metabolic labeling with [32P]Pi and affinity chromatography using anti-phosphotyrosine antibodies. The other cannot be detected by this technique but can be detected among the proteins which bind to the antibodies by in vitro iodination with 125I. Presumably proteins of the second class contain phosphotyrosine at which the phosphate undergoes very slow turnover. The incubation of cells in phosphate-minus medium caused a marked reduction in the levels of phosphotyrosine-containing proteins, this explaining the failure of detection of the second class proteins even after prolonged labeling with [32P]Pi.