Abstract
P36 is a major substrate of the tyrosine protein kinases. P36 isolated from bovine intestine was used in phosphorylation reactions with pp60src. Phosphorylation was stimulated 3–5-fold by Ca2+, however the K m was the same (2.5μM) at high or low Ca2+. Although the level of free Ca2+ needed for this enhanced phosphorylation was 10−4–10−3)−3 M, phosphatidylserine shifted the Ca2+ sensitivity to the 10−6–10−5 M range. Independent evidence suggested that p36 interacts directly with liposomes containing phosphatidylserine. This raises the possibility that p36, like c-kinase, is a Ca2+-activated, phospholipid-dependent protein.

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