Localization and Function of Cyclin B1 and Cyclin B2 during Porcine Oocyte Maturation

Oocyte maturation is regulated by maturation/M-phase promoting factor (MPF), a crucial M-phase regulating enzyme composed of a catalytic subunit, p34 cdc2 , and a regulatory subunit, cyclin B. The amount of p34 cdc2 is almost constant during oocyte maturation, and the amount of cyclin B is the principal factor determining MPF activity [1]. The presence of two types of cyclin B, cyclin B1 and cyclin B2, has been shown in vertebrates. In human cells, cyclin B1 can cause chromosome condensation, reorganization of the microtubules, and disassembly of the nuclear lamina and of the Golgi apparatus, whereas the role of cyclin B2 is restricted only to disassembly of the Golgi apparatus [2, 3]. In maturing oocytes, differences between cyclin B1 and cyclin B2 functions have been reported in the first meiotic spindle formation and the second metaphase arrest in frog and mouse oocytes, respectively [4(cid:150)6]. In our laboratory, we have studied cyclin B functions during porcine oocyte maturation for the past several years. The present review describes our recent observations with regard to protein levels, intracellular localizations and roles of cyclin B. We focus here on the differences between cyclin B1 and cyclin B2. density of