The role of cyclin B in meiosis I.
In clams, fertilization is followed by the prominent synthesis of two cyclins, A and B. During the mitotic cell cycles, the two cyclins are accumulated and then destroyed near the end of each metaphase. Newly synthesized cyclin B is complexed with a small set of other proteins, including a kinase that phosphorylates cyclin B in vitro. While both cyclins can act as general inducers of entry into M phase, the two are clearly distinguished by their amino acid sequences (70% nonidentity) and by their different modes of expression in oocytes and during meiosis. In contrast to cyclin A, which is stored solely as maternal mRNA, oocytes contain a stockpile of cyclin B protein, which is stored in large, rapidly sedimenting aggregates. Fertilization results in the release of cyclin B to a more disperse, soluble form. Since the first meiotic division in clams can proceed even when new protein synthesis is blocked, these results strongly suggest it is the fertilization-triggered unmasking of cyclin B protein that drives cells into meiosis I. We propose that the unmasking of maternal cyclin B protein allows it to interact with cdc2 protein kinase, which is also stored in oocytes, and that the formation of this cyclin B/cdc2 complex generates active M phase-promoting factor.
Westendorf
J M
JM
Program in Cell and Developmental Biology, Harvard Medical School, Boston, Massachusetts 02115.
Swenson
K I
KI
Ruderman
J V
JV
D000595:Amino Acid Sequence; D000818:Animals; D001483:Base Sequence; D049872:Bivalvia / Q000196:embryology; D016213:Cyclins; D004247:DNA / Q000235:genetics; D004625:Embryo, Nonmammalian / Q000166:cytology / Q000502:physiology; D005260:Female; D007447:Invertebrate Hormones / Q000235:genetics / Q000502:physiology*; D008540:Meiosis; D008969:Molecular Sequence Data; D009865:Oocytes / Q000378:metabolism; D011494:Protein Kinases / Q000235:genetics / Q000378:metabolism; D012333:RNA, Messenger / Q000235:genetics; D014981:Xenopus
D016213:Cyclins; D007447:Invertebrate Hormones; D012333:RNA, Messenger; D004247:DNA; D011494:Protein Kinases