Xenopus laevis oocyte calmodulin in the process of meiotic maturation

Calcium ions are postulated to be involved in the process of meiotic maturation of amphibian oocytes. Since several of the calcium effects in eukaryotic cells are mediated by calmodulin, the present study was undertaken to assess the presence and level of calmodulin in Xenopus laevis oocytes before and after progesterone treatment. Calmodulin was shown to be present at a concentration of approximately microM in control oocytes cytosol. This level remained stable for 2 h when the maturation promoting factor appeared, and increased to approximately 44 to 59 microM at the time of the germinal vesicle breakdown. Maturation is therefore associated with calmodulin synthesis. Xenopus calmodulin was isolated from oocyte cytosol after heat treatment, anion exchange chromatography, and gel filtration, with a yield of approximately 23%. When compared to mammalian calmodulins, the amphibian protein exhibited the same ultraviolet absorption spectrum, a similar amino acid composition with 1 residue of trimethyllsine, and the same shape conformers in the absence or presence of divalent metals, as shown by different mobilities upon dodecyl sulfate-polyacrylamide gel electrophoresis. The protein migrated faster in the presence than in the absence of Ca2+ ions, Mn2+ and Mg2+ being less effective. It was able to activate calmodulin-deficient myosin light chain kinase. Its high serine content and the tryptic peptide maps obtained after high performance liquid chromatography point, however, to minor differences in the primary structures of mammalian and amphibian calmodulins.