SBP1 Genetically Interacts With WHI3, a Cell Cycle Control Gene in Saccharyomyces cerevisiae

Proper control of cell cycle is crucial allows cells to grow to the appropriate size in the budding yeast, Saccharyomyces cerevisiae , and to ensure cancer formation does not occur in humans. Although there are a number of cell cycle checkpoints which control passage through the cell cycle, the most critical of these is the G1/S checkpoint, also known as START. Upon passage through the G1/S checkpoint, the cell is committed to replicate its DNA and undergo a round of cell division. In yeast, expression of the cyclin Cln3p, a mammalian cyclin homolog, is necessary to progress through the G1/S checkpoint. Cln3p accumulates to high levels during late G1phase and forms a complex with Cdc28p. The Cln3p/CDC28 complex then enters the nucleus and drives the cell through the G1/S checkpoint. In order to control Cln3p levels through early and middle G1 phase, such that the cell proceeds through the G1/S checkpoint at the appropriate time, it is thought that Whi3p, an RNA recognition motif (RRM) containing protein, translationally represses the CLN3 mRNA. Recently, a genetic interaction was identified between WHI3 and SBP1. SBP1 encodes a single stranded RNA binding protein Sbp1p which has been shown to work in conjunction with Dhh1p repress global mRNA translation under physiological stress. Therefore, it is possible Whi3p may work in conjunction with Sbp1p to translationally repress the CLN3 mRNA during early G1 phase. In this work, we show that overexpression of Sbp1p has a significant effect in whi3Δ cells. First, overexpression of Sbp1p in whi3Δ cells results in a significant growth defect at 37° C. Second, overexpression of Sbp1p in whi3Δ cells results in increased cell size as compared to whi3Δ cells. Next, we will investigate whether Sbp1p is necessary for CLN3 mRNA translational repression.