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Molecular and Cellular Biology, August 1999, p. 5512-5522, Vol. 19, No. 8
Institute of Molecular and Cell Biology,
Singapore 117609, Singapore
Received 4 December 1998/Returned for modification 27 January
1999/Accepted 10 May 1999
Saccharomyces cerevisiae proteins Cdc4 and Cdc20
contain WD40 repeats and participate in proteolytic processes. However,
they are thought to act at two different stages of the cell cycle: Cdc4
is involved in the proteolysis of the Cdk inhibitor, Sic1, necessary
for G1/S transition, while Cdc20 mediates
anaphase-promoting complex-dependent degradation of anaphase inhibitor
Pds1, a process necessary for the onset of chromosome segregation. We
have isolated three mutant alleles of CDC4
(cdc4-10, cdc4-11, and cdc4-16)
which suppress the nuclear division defect of cdc20-1
cells. However, the previously characterized mutation
cdc4-1 and a new allele, cdc4-12, do not
alleviate the defect of cdc20-1 cells. This genetic interaction suggests an additional role for Cdc4 in G2/M.
Reexamination of the cdc4-1 mutant revealed that, in
addition to being defective in the onset of S phase, it is also
defective in G2/M transition when released from
hydroxyurea-induced S-phase arrest. A second function for
CDC4 in late S or G2 phase was further
confirmed by the observation that cells lacking the CDC4
gene are arrested both at G1/S and at G2/M. We
subsequently isolated additional temperature-sensitive mutations in the
CDC4 gene (such as cdc4-12) that render the
mutant defective in both G1/S and G2/M
transitions at the restrictive temperature. While the G1/S
block in both cdc4-12 and cdc4
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Copyright © 1999, American Society for Microbiology. All rights reserved.
Cdc4, a Protein Required for the Onset of S
Phase, Serves an Essential Function during G2/M
Transition in Saccharomyces cerevisiae
mutants is
abolished by the deletion of the SIC1 gene (causing the
mutants to be arrested predominantly in G2/M), the preanaphase arrest in the cdc4-12 mutant is relieved by the
deletion of PDS1. Collectively, these observations suggest
that, in addition to its involvement in the initiation of S phase, Cdc4
may also be required for the onset of anaphase.
*
Corresponding author. Mailing address: Institute of
Molecular and Cell Biology, 30 Medical Dr., Singapore 117609, Singapore. Phone: (65) 774 3612 or 874 6680. Fax: (65) 779 1117. E-mail: mcbucs{at}imcb.nus.edu.sg.
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