Tea4p links microtubule plus ends with the formin for3p in the establishment of cell polarity.
Microtubules regulate actin-based processes such as cell migration and cytokinesis, but molecular mechanisms are not understood. In the fission yeast Schizosaccharomyces pombe, microtubule plus ends regulate cell polarity in part by transporting the kelch repeat protein tea1p to cell ends. Here, we identify tea4p, a SH3 domain protein that binds directly to tea1p. Like tea1p, tea4p localizes to growing microtubule plus ends and to cortical sites at cell ends, and it is necessary for the establishment of bipolar growth. Tea4p binds directly to and recruits the formin for3p, which nucleates actin cable assembly. During "new end take off" (NETO), formation of a protein complex that includes tea1p, tea4p, and for3p is necessary and sufficient for the establishment of cell polarity and localized actin assembly at new cell ends. Our results suggest a molecular mechanism for how microtubule plus ends regulate the spatial distribution of actin assembly.