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* Department of Biology and Geosciences, Faculty of Science, Shizuoka University, Shizuoka 422, 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.
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
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