Dbf4p, an Essential S Phase-Promoting Factor, Is Targeted for Degradation by the Anaphase-Promoting Complex

Abstract

The Dbf4p/Cdc7p protein kinase is essential for the activation of replication origins during S phase. The catalytic subunit, Cdc7p, is present at constant levels throughout the cell cycle. In contrast, we show here that the levels of the regulatory subunit, Dbf4p, oscillate during the cell cycle. Dbf4p is absent from cells during G₁and accumulates during the S and G₂ phases. Dbf4p is rapidly degraded at the time of chromosome segregation and remains highly unstable during pre-Start G₁ phase. The rapid degradation of Dbf4p during G₁ requires a functional anaphase-promoting complex (APC). Mutation of a sequence in the N terminus of Dbf4p which resembles the cyclin destruction box eliminates this APC-dependent degradation of Dbf4p. We suggest that the coupling of Dbf4p degradation to chromosome separation may play a redundant role in ensuring that prereplicative complexes, which assemble after chromosome segregation, do not immediately refire.

ACKNOWLEDGMENTS

We thank Tamara Tugal for advice on phosphatase treatment and Hiro Yamano and Tim Hunt for discussions.

M.G.F. gratefully acknowledges the support of the Gulbenkian Ph.D. Program in Biology and Medicine.

ADDENDUM IN PROOF

While this paper was under consideration, two papers (L. Cheng et al., Mol. Cell. Biol. 19:4270–4278, 1999, and G. Oshiro et al., Mol. Cell. Biol. 19:4888–4896, 1999) showing APC-dependent degradation of Dbf4p have been published.