Regulation of cell size and Wee1 by elevated levels of Cdr2

Abstract

Many cell cycle regulatory proteins catalyze cell cycle progression in a concentration-dependent manner. In fission yeast S. pombe, the protein kinase Cdr2 promotes mitotic entry by organizing cortical oligomeric nodes that lead to inhibition of Wee1, which itself inhibits Cdk1. cdr2{Delta} cells lack nodes and divide at increased size due to overactive Wee1, but it has not been known how increased Cdr2 levels might impact Wee1 and cell size. Using a Tetracycline-inducible expression system, we found that a 6X increase in Cdr2 expression caused hyperphosphorylation of Wee1 and reduction in cell size. This overexpressed Cdr2 formed clusters that sequestered Wee1 adjacent to the nuclear envelope. Cdr2 mutants that disrupt either kinase activity or clustering ability failed to sequester Wee1 and to reduce cell size. We propose that Cdr2 acts as a dosage-dependent regulator of cell size by sequestering its substrate Wee1 away from Cdk1 in the nucleus. This mechanism has implications for other clustered kinases, which may act similarly by sequestering substrates.