Elongation factor-2 in chick embryo is phosphorylated on tyrosine as well as serine and threonine.

An endogenous 95 kDa chick embryo cytosolic protein (p95) was phosphorylated in the presence of [gamma-32P]ATP and the kinase activity for p95 was mostly associated with particulate fraction. Phosphorylation of p95 was prominent in embryos of early developmental stage. Hydrolysis of p95 phosphoprotein yielded phosphotyrosine in addition to phosphothreonine and phosphoserine. Native p95 was also tyrosine-phosphorylated. p95 phosphoprotein was purified by DEAE-Sephacel chromatography and immunoprecipitation with anti-phosphotyrosine antibody and the amino acid sequence was determined. The N-terminal sequence, Val-Asn-Phe-Thr-Val-Asp-Gln-Ile-Arg-Ala-Ile-Met-Asp- Lys-Lys-Ala-Asn-Ile-Arg-Asn-Met-, was found to be identical to those of elongation factor-2 (EF-2) of both rat and hamster. Our results suggest the presence of other EF-2 kinase in chick embryo cell than the previously reported Ca2+/calmodulin-dependent protein kinase III.