Oligomerization and exocyst coupling underlie Spa2-mediated focusing of polarized growth in fission yeast

Polarized cell growth in fungi requires the spatial restriction of exocytosis to discrete cortical domains. Defined by a characteristic domain architecture, the evolutionarily conserved scaffold protein Spa2 localizes to sites of polarized growth in fungi and has been implicated in morphogenic processes including hyphal extension in filamentous fungi and budding yeast mating. Schizosaccharomyces pombe is a well-studied and powerful model organism for elucidating mechanisms of polarized growth. However, identifying a role for Spa2 in S. pombe morphogenesis has been elusive, highlighting a gap in defining a broadly conserved Spa2 function. Here, we undertook a comprehensive and comparative dissection of the targeting mechanisms, interactome and function of Spa2 in S. pombe. We find that all of the conserved domains in Spa2 influence Spa2 localization to sites of polarized growth in an exocyst-dependent and largely cytoskeleton-independent manner. At cell tips, stable complexes of oligomerized Spa2 contribute to constraining the growth zone, in part by delivering the Rab GTPase-activating protein for the Sec4 homolog Ypt2. Despite species-specific wiring of Spa2 protein networks, our results underscore an evolutionarily conserved role for Spa2 in sharpening the spatial focus of polarized growth.