Memory of growth promotes early Ras GTPase activation to establish dominance of inherited cell ends in Schizosaccharomyces pombe.

Many eukaryotic cells maintain their polarity pattern and shape across multiple rounds of division. This requires that the daughter cells re-establish polarity at the correct site after cell division. Fission yeast cells (Schizosaccharomyces pombe) are bipolar; however, after division, growth initiates first at the old end of the cell that was inherited from the mother. This old-end dominance is dependent on that end having grown in the previous generation, suggesting that cells must recognize when an end has a history of growth. We find that this memory of growth is imparted by the growth-dependent localization of Rax1 (encoded by SPAC23G3.05c) and Rax2 proteins. Rax1 and Rax2 promote early activation of Ras1 GTPase at dividing cell ends by recruitment of its guanine nucleotide exchange factor Efc25. This enables timely transition of Cdc42 activity from the division site to the old end, thus giving this end an advantage in polarized growth. Consistently, modulating Ras1 activation via disruption of its regulators Efc25 and Gap1 leads to corresponding changes in old-end dominance. Here, we demonstrate the importance of memory of growth in maintaining persistent sites of growth from one generation to the next.