High dosage expression of a zinc finger protein, Grt1, suppresses a mutant of fission yeast slp1(+), a homolog of CDC20/p55CDC/Fizzy.

Selective proteolysis at and after the onset of anaphase is a key cell cycle event required for sister chromatid separation as well as for exit from mitosis. It requires ubiquitination of substrates by Anaphase Promoting Complex(APC)/Cyclosome. Slp1, a WD-repeat protein, is a putative activator for APC in fission yeast. With another WD- repeat protein, Ste9/Srw1, it is thought to promote the proteolysis in a substrate-specific manner. We report here characterization of a temperature-sensitive (ts) slp1 mutant and its high-dosage suppressor, grt1(+). In cells arrested in metaphase, wild-type Slp1 was preferentially found in a complex with hyperphosphorylated Cut9 (subunit of APC), whereas the ts Slp1 protein, lacking the last 113 amino acids, failed to interact with Cut9. The temperature sensitivity was suppressed by high dosage expression of a zinc finger protein, Grt1. The ts slp1 mutant was unable to maintain the normal level of Grt1 protein. The reduction in the Grt1 level may be a primary defect since high dosage expression of grt1(+) rescues the slp1 mutant. The grt1-suppression had an additive effect to ste9 and wee1-50, both of which partially suppress the ts slp1 mutant. Therefore, grt1(+) would define an independent pathway that facilitates the function of Slp1.