We found that in starfish oocytes two different enzymes, phosphatidylserine synthase-1 (PSS1) and -2 (PSS2), which synthesize phosphatidylserine by a base-exchange reaction, are present. We studied phosphatidylserine synthesis in immature oocytes which still contain the nucleus (germinal vesicles) and in mature cells, in which the re-initiation of the meiotic cycle induced by the hormone 1-methyladenine led to structural changes in the endoplasmic reticulum, to the disappearance of the nuclear envelope and to the intermixing of the nucleoplasm with the cytoplasm. It was found that the levels of PSS1 and PSS2 transcripts were higher in immature and mature oocytes, respectively. The level of the expressed PSS2 protein, higher than that of PSS1, was not influenced by the maturation process, whereas the level of PSS1 protein was higher in immature than in mature oocytes. Serine incorporation into phosphatidylserine was enhanced in immature oocytes. The depletion of calcium stores by thapsigargin resulted in 50% lowering of phosphatidylserine synthesis. We suggest that changes in phosphatidylserine synthesis may be affected by the release of calcium stored in the nuclear envelope and in the endoplasmic reticulum, the membranes that undergo disintegration and fragmentation during meiosis. The reason for the greater synthesis of PS may be the higher level of expression of PSS1 in immature oocytes.
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