The role of microtubules and inositol triphosphate induced Ca2+ release in the tyrosine phosphorylation of mitogen-activated protein kinase in extracts of Xenopus laevis oocytes

Microsomal fractions of Xenopus oocytes release preloaded Ca-45(2+) when treated with inositol triphosphate (InsP(3)). The effective concentration of InsP(3) required for half-maximal release (EC(50)) is 59 nM and maximal release occurs at similar to 2 mu M InsP(3). Uptake and release of Ca-45(2+) are not altered by the catalytic subunit of protein kinase A, dibutyrl cyclic adenosine monophosphate, protein kinase A peptide inhibitor or nocodazole. In contrast, taxol decreases the sensitivity of the microsomal fraction to InsP(3), shifting the EC(50) for InsP(3)-induced Ca2+ release from 59 to 259 nM. In lysates of oocytes, InsP(3)-induced Ca2+ release causes the tyrosine phorphorylation of a 42000 (M(r) 42k) protein identified as 42k mitogen-activated protein (MAP) kinase. InsP(3)-induced tyrosine phosphorylation of MAP kinase is prevented by BAPTA and taxol, but not by nocodazole. Thus, microtubule polymerisation modifies InsP(3)-induced Ca2+ release, thereby inhibiting phosphorylation of MAP kinase.