Analysis of the mechanism responsible for cell cycle reentry during mating pheromone response in S.cerevisiae

ALICIA GRANDE; EDGAR ALTSZYLER; ARIEL CHERNOMORETZ; ALEJANDRO COLMAN-LERNER

Rosario

Congreso; SAIB 2014 Congress; 2014

Sociedad Argentina de Investigación Bioquímica

In haploid yeast, mating pheromone triggers a fate decision: to arrest of the cell cycle in G1 and initiate mating events. However, the decision depends on pheromone concentration. Cells exposed to low pheromone, uniformly ignore its presence and continue to bud and cells exposed to very high concentrations uniformly arrest and from mating projections termed shmoos. At intermediate doses (near the KD of the pheromone receptor), these two phenotypes coexist and yet a third, intermediate phenotype appears: cells respond for some time, as judged by the development of mating projections, but, after a variable period, they reenter the cell cycle (ie, bud). Here, we focused on elucidating the mechanism underlying the ?switching? behavior. We believe that the explanation lies in the interaction between the pheromone pathway and the cell division cycle. Therefore, we built a simple mechanistic mathematical model based on current literature, with a small number of parameters and dynamical variables The model is based on the assumption that the key interactions are the inhibition of the G1 Cdk (Cdc28/Cln1,2,3) by the CKI activated by pheromone, Far1 and the block of the pheromone response by active G1 Cdks. The winner of this ?tug of war? decides the cell fate. As a first approximation, we used a set of nonlinear ordinary differential equations (ODE) that describe the dynamics of the simultaneously occurring reactions. The model thus simulated is able to explain the switching cells behavior. A core assumption of the model is that a positive feedback in the synthesis of the G1 cyclins Cln1 and Cln2 provides the necessary non-linearity in the system of cause the observed behavior. We tested this supposition experimentally, using the appropriate deletion yeast strains and found that removal of this feedback did not diminish the number of ?switching? cells. This key experiment allowed us to reject the model and the theory behind it. We are now exploring another hypothesis that involves an incoherent feedforward loop in the system. In general, our work illustrates the usefulness of models as tools for the understating of complex cellular behaviors.