Phosphorylation of p36 in vitro with pp60 src

P36 is a major substrate of the tyrosine protein kinases. P36 isolated from bovine intestine was used in phosphorylation reactions with pp60 src. Phosphorylation was stimulated 3–5-fold by Ca 2+, however the Kmwas the same (2.5μM) at high or low Ca 2+. Although the level of free Ca 2+needed for this enhanced phosphorylation was 10 −4–10 −3) −3M, phosphatidylserine shifted the Ca 2+sensitivity to the 10 −6–10 −5M range. Independent evidence suggested that p36 interacts directly with liposomes containing phosphatidylserine. This raises the possibility that p36, like c-kinase, is a Ca 2+-activated, phospholipid-dependent protein.