Src family tyrosine kinases regulate oocyte maturation and fertilization in many species, yet their physiological roles in Xenopus laevis (X. laevis) remain incompletely defined. Here, we generated three X. laevis Src (xSrc) constructs with defined point mutations allowing for selective immunochemical detection and controlled modulation of kinase activity: wild type (xSrcWT, Arg121His), constitutively active (xSrcKA, Arg121His/Tyr526Phe), and kinase-negative (xSrcKN, Arg121His/Lys294Met). Capped mRNAs were microinjected into immature oocytes, and effects on meiotic maturation and egg activation were analyzed. All constructs produced detectable Src protein within 4–5 h after injection without inducing progesterone-independent maturation. Following progesterone treatment, MAP kinase phosphorylation, CDK1 activation, and germinal vesicle breakdown (GVBD) occurred normally in all groups, although xSrcKA-expressing oocytes showed a modest but reproducible acceleration of MAPK activation and GVBD. Global tyrosine phosphorylation analysis revealed increased phosphorylation of several proteins, including a prominent ~50 kDa substrate, specifically in xSrcKA oocytes. After maturation, oocytes were subjected to artificial activation. xSrcKN-expressing oocytes responded normally to Ca2+ ionophore (A23187), indicating that Src activity is not required for direct Ca2+-mediated activation. In contrast, xSrcKN oocytes exhibited markedly reduced activation in response to hydrogen peroxide or Cathepsin B, which stimulate membrane-associated signaling pathways. These findings demonstrate that Src kinase activity is required for membrane signal-mediated egg activation but is dispensable for activation driven by direct intracellular Ca2+ elevation. Collectively, our results identify Src kinase as a positive regulator of progesterone-induced meiotic maturation and a critical mediator of specific fertilization-like activation pathways in X. laevis.
Sato et al. (Fri,) studied this question.