Spatial controls for growth zone formation during the fission yeast cell cycle

Because of its regular shape, fission yeast is becoming an increasingly important organism in the study of cellular morphogenesis. Genetic experiments with mutants and drug treatment studies with wild-type cells have revealed the importance of microtubules in controlling new growth zone formation. It is believed that microtubules exert this role by delivering to cell ends a 'dynamic landmark' protein, tea1p, which promotes actin polymerization and growth zone formation. Here we present a simple model for fission yeast morphogenesis that describes the interplay between these two cytoskeletal elements. An essential assumption of the model is that actin polymerization is a self-reinforcing process: filamentous actin promotes its own formation from globular actin subunits via regulatory molecules. In our model, microtubules stimulate actin polymerization by delivering a component of the autocatalytic actin-assembly feedback loop (not by delivering a de novo inducer of actin polymerization). We show that the model captures all the characteristic features of polarized growth in fission yeast during normal mitotic cycles. We categorize the types of growth patterns that can exist in the model and show that they correspond to the major classes of morphogenetic mutants (monopolar, orb, banana and tea). Based on these results, we propose that fission yeast cells have specific size ranges in which they can exhibit two or more different stable patterns of growth.