THE ROLE OF MICROTUBULES AND INOSITOL TRIPHOSPHATE INDUCED CA2-ACTIVATED PROTEIN-KINASE IN EXTRACTS OF XENOPUS-LAEVIS OOCYTES( RELEASE IN THE TYROSINE PHOSPHORYLATION OF MITOGEN)

Microsomal fractions of Xenopus oocytes release preloaded Ca-45(2+) wh en treated with inositol triphosphate (InsP(3)). The effective concent ration 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 re lease of Ca-45(2+) are not altered by the catalytic subunit of protein kinase A, dibutyrl cyclic adenosine monophosphate, protein kinase A p eptide inhibitor or nocodazole. In contrast, taxol decreases the sensi tivity 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 i s prevented by BAPTA and taxol, but not by nocodazole. Thus, microtubu le polymerisation modifies InsP(3)-induced Ca2+ release, thereby inhib iting phosphorylation of MAP kinase.