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Vol. 19, Issue 3, 1032-1045, March 2008

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Ypi1, a Positive Regulator of Nuclear Protein Phosphatase Type 1 Activity in Saccharomyces cerevisiae

Department of Biochemistry and Molecular Biology, Louisiana State University Health Sciences Center, Shreveport, LA 71130

Submitted May 25, 2007; Revised November 21, 2007; Accepted December 20, 2007
Monitoring Editor: Charles Boone

The catalytic subunit of protein phosphatase type 1 (PP1) has an essential role in mitosis, acting in opposition to the Ipl1/Aurora B protein kinase to ensure proper kinetochore-microtubule interactions. However, the regulatory subunit(s) that completes the PP1 holoenzyme that functions in this capacity is not known. We show here that the budding yeast Ypi1 protein is a nuclear protein that functions with PP1 (Glc7) in this mitotic role. Depletion of cellular Ypi1 induces mitotic arrest due to activation of the spindle checkpoint. Ypi1 depletion is accompanied by a reduction of nuclear PP1 and by loss of nuclear Sds22, a Glc7 binding partner that is found in a ternary complex with Ypi1 and Glc7. Expression of a Ypi1 variant that binds weakly to PP1 also activates the spindle checkpoint and suppresses the temperature sensitivity of an ipl1-2 mutant. These results, together with genetic interactions among YPI1, GLC7, and SDS22 mutants, indicate that Ypi1 and Sds22 are positive regulators of the nuclear Glc7 activity that is required for mitosis.

* Present address: Laboratory of Human Retrovirology, Clinical Services Program, Applied and Developmental Research Support Program, Science Application International Corporation (SAIC)-Frederick Inc., National Cancer Institute at Frederick, Frederick, MD 21702.

Address correspondence to: Kelly Tatchell (ktatch{at}lsuhsc.edu)

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