A Model of Fission-Yeast Cell Shape Driven by Membrane-Bound Growth Factors

Abstract

Fission yeast serves as a model for how cellular polarization machinery is used to regulate cell growth. Many studies identify active Cdc42, found in a cap at the inner membrane of growing tips, as an important growth regulator, likely through control of exocyst tethering and activation of actin-cable nucleators. To investigate how these molecular processes might control shape, we propose a simple model based on the hypotheses that (i) the delivery and internalization rate of wall or membrane components limits cell expansion and (ii) a growth factor, such as Cdc42, signals for delivery of these components. We numerically simulate cell growth according to an axisymmetric, finite-element computational model that couples growth-factor-directed orthogonal expansion of the cell membrane and cell-wall remodeling to reaction and diffusion of the growth factor on that membrane. We explore limiting conditions for polarized growth and consider the additional effects of membrane elasticity and flow. We find a relationship between cap size and diameter, and motivate future experiments on the link between cell signaling and shape. Fission-yeast Cdc42 is regulated by a number of proteins whose absence lead to defects in shape or polarized growth, such as cells of varying diameter, round cells, and branched cells. We consider the roles of auxiliary proteins, incorporate findings on length-dependent polarity change, compare model results to cell morphologies of mutants of Cdc42 regulators, and suggest possible mechanistic roles for these regulators. An extension to three dimensions adds the capacity to reproduce bent-cell morphology and to investigate the stability of axisymmetric solutions.