A high affinity polyclonal antibody specific for phosphotyrosyl residues has been used in immunoblotting experiments to survey developing embryonic chicken tissues for the presence and characteristics of tyrosine phosphorylated proteins. Proteins phosphorylated on tyrosine were found to be present in all the embryonic tissues examined, including heart, thigh, gizzard, intestine, lung, liver, kidney, brain, and lens, from 7 to 21 d of development in ovo, but were greatly reduced or absent in the same tissues taken from adult chickens. A limited number of major tyrosine phosphorylated proteins were seen in all the tissues examined and they ranged in molecular mass from 35 to 220 kD. Most of the tissues contained proteins phosphorylated on tyrosine with apparent molecular masses of 120, 70, 60, and 35 kD, suggesting that the substrates of tyrosine protein kinases in different tissues may be related proteins. One-dimensional peptide mapping of the 120- and 70-kD protein bands indicated a close structural relationship among the phosphotyrosine-containing proteins of 120 kD, and similarly among those of 70 kD, from the different tissues.

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