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

Abstract

SummaryMicrosomal fractions ofXenopusoocytes release preloaded45Ca2+when treated with inositol triphosphate (InsP3). The effective concentration of InsP3required for half-maximal release (EC50) is 59 nM and maximal release occurs at ∼ 2 μM InsP3. Uptake and release of45Ca2+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 InsP3, shifting the EC50for InsP3-induced Ca2+release from 59 to 259 nM. In lysates of oocytes, InsP3-induced Ca2+release causes the tyrosine phorphorylation of a 42000 (Mr42k) protein identified as 42k mitogen-activated protein (MAP) kinase. InsP3-induced tyrosine phosphorylation of MAP kinase is prevented by BAPTA and taxol, but not by nocodazole. Thus, microtubule polymerisation modifies InsP3-induced Ca2+release, thereby inhibiting phosphorylation of MAP kinase.