Rax1/2 promote memory of growth by early Ras1 activation to maintain polarity across generations

Abstract

As polarized cells divide, daughter cells must re-establish polarity at the correct site to maintain their shape. Fission yeast cells are bipolar, however growth initiates first at the old end of the cell that was inherited from the mother. This dominance of the old end is dependent on that end having grown in the previous generation, suggesting cells must be able to recognize when an end has a previous history of growth. We find that the proteins Rax1/2 act as markers for memory of growth by localizing to growing cell ends and remaining through division. Rax1/2 then promote activation of Ras1-GTPase at dividing cell ends by recruitment of its GEF Efc25. Localized Ras1 activity at the old ends ensures timely activation of the Rho GTPase Cdc42 to initiate polarized growth, providing an advantage to the old end. Through this work, we demonstrate the role of a Ras-GTPase in positioning the site of polarization and maintaining cell shape across multiple generations.