Originally published as Genetics Published Articles Ahead of Print on May 4, 2007.

Genetics, Vol. 176, 1541-1555, July 2007, Copyright © 2007
doi:10.1534/genetics.107.073494

Phosphorylation of the Sic1 Inhibitor of B-Type Cyclins in Saccharomyces cerevisiae Is Not Essential but Contributes to Cell Cycle Robustness

Frederick R. Cross1, Lea Schroeder and James M. Bean2

The Rockefeller University, New York, New York 10021

1 Corresponding author: The Rockefeller University, 1230 York Ave., New York, NY 10021.
E-mail: fcross{at}rockefeller.edu

In budding yeast, B-type cyclin (Clb)-dependent kinase activity is essential for S phase and mitosis. In newborn G1 cells, Clb kinase accumulation is blocked, in part because of the Sic1 stoichiometric inhibitor. Previous results strongly suggested that G1 cyclin-dependent Sic1 phosphorylation, and its consequent degradation, is essential for S phase. However, cells containing a precise endogenous gene replacement of SIC1 with SIC1-0P (all nine phosphorylation sites mutated) were fully viable. Unphosphorylatable Sic1 was abundant and nuclear throughout the cell cycle and effectively inhibited Clb kinase in vitro. SIC1-0P cells had a lengthened G1 and increased G1 cyclin transcriptional activation and variable delays in the budded part of the cell cycle. SIC1-0P was lethal when combined with deletion of CLB2, CLB3, or CLB5, the major B-type cyclins. Sic1 phosphorylation provides a sharp link between G1 cyclin activation and Clb kinase activation, but failure of Sic1 phosphorylation and proteolysis imposes a variable cell cycle delay and extreme sensitivity to B-type cyclin dosage, rather than a lethal cell cycle block.