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© The Rockefeller University Press, 0021-9525/1997//1313 $5.00
The Journal of Cell Biology, Volume 138, Number 6, , 1997 1313-1322

Article

Initiation of Cyclin B Degradation by the 26S Proteasome upon Egg Activation



Toshinobu Tokumoto*, Masakane Yamashita{ddagger}, Mika Tokumoto§, Yoshinao Katsu§, Ryo Horiguchi*, Hiroko Kajiura§, and Yoshitaka Nagahama§

* Department of Biology and Geosciences, Faculty of Science, Shizuoka University, Shizuoka 422, Japan; {ddagger} Division of Biological Sciences, Graduate School of Science, Hokkaido University, Sapporo 060, Japan; and § Laboratory of Reproductive Biology, National Institute for Basic Biology, Okazaki 444, Japan

Immediately before the transition from metaphase to anaphase, the protein kinase activity of maturation or M-phase promoting factor (MPF) is inactivated by a mechanism that involves the degradation of its regulatory subunit, cyclin B. The availability of biologically active goldfish cyclin B produced in Escherichia coli and purified goldfish proteasomes (a nonlysosomal large protease) has allowed the role of proteasomes in the regulation of cyclin degradation to be examined for the first time. The 26S, but not the 20S proteasome, digested recombinant 49-kD cyclin B at lysine 57 (K57), producing a 42-kD truncated form. The 42-kD cyclin was also produced by the digestion of native cyclin B forming a complex with cdc2, a catalytic subunit of MPF, and a fragment transiently appeared during cyclin degradation when eggs were released from metaphase II arrest by egg activation. Mutant cyclin at K57 was resistant to both digestion by the 26S proteasome and degradation at metaphase/anaphase transition in Xenopus egg extracts. The results of this study indicate that the destruction of cyclin B is initiated by the ATP-dependent and ubiquitin-independent proteolytic activity of 26S proteasome through the first cutting in the NH2 terminus of cyclin (at K57 in the case of goldfish cyclin B). We also surmise that this cut allows the cyclin to be ubiquitinated for further destruction by ubiquitin-dependent activity of the 26S proteasome that leads to MPF inactivation.

Please address all correspondence to Dr. Y. Nagahama, Laboratory of Reproductive Biology, National Institute for Basic Biology, Okazaki 444, Japan. Tel.: (81) 564-55-7550; Fax: 81-564-55-7556; E-mail: nagahama{at}nibb.ac.jp

1. Abbreviation used in this paper: MPF, maturation or M-phase promoting factor.


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