Rgf1 GEF activity toward Rho1 defines a new actin-dependent signal to determine growth sites independently of microtubules and Tea1

Abstract

Cellular asymmetry begins with the selection of a discrete point on the cell surface that triggers Rho-GTPases activation and localized assembly of the cytoskeleton to establish new growth zones. The cylindrical shape of fission yeast is organized by microtubules that deliver the landmark Tea1–Tea4 complex at the cell tips to define the growth poles. However, only a fewtea1Δ cells mistaken the direction of growth, indicating that they manage to detect their growth sites. Here we show that Rgf1 (Rho1-GEF) and Tea4 are components of the same complex and that Rgf1 activity toward Rho1 is required for strengthen Tea4 at the cell tips. Moreover, in cells lacking Tea1, selection of the correct growth site depends on Rgf1 and on a correctly polarized actin cytoskeleton, both necessary for Rho1 activation at the pole. We propose an actin-dependent mechanism driven by Rgf1–Rho1 that marks the poles independently of microtubules and the Tea1–Tea4 complex.