A noncanonical GTPase signaling mechanism controls exit from mitosis in budding yeast

In the budding yeast Saccharomyces cerevisiae, exit from mitosis is coupled to spindle position to ensure successful genome partitioning between mother and daughter cells. This coupling occurs through a GTPase signaling cascade known as the mitotic exit network (MEN). The MEN senses spindle position via a Ras-like GTPase Tem1 which localizes to the spindle pole bodies (SPBs, yeast equivalent of centrosomes) during anaphase and signals to its effector protein kinase Cdc15. How Tem1 couples the status of spindle position to MEN activation is not fully understood. Here, we show that Cdc15 has a relatively weak preference for Tem1^GTP and Tem1's nucleotide state does not change upon MEN activation. Instead, we find that Tem1's nucleotide cycle establishes a localization-based concentration difference in the cell where only Tem1^GTP is recruited to the SPB, and spindle position regulates the MEN by controlling Tem1 localization to the SPB. SPB localization of Tem1 primarily functions to promote Tem1-Cdc15 interaction for MEN activation by increasing the effective concentration of Tem1. Consistent with this model, we demonstrate that artificially tethering Tem1 to the SPB or concentrating Tem1 in the cytoplasm with genetically encoded multimeric nanoparticles could bypass the requirement of Tem1^GTP and correct spindle position for MEN activation. This localization/concentration-based GTPase signaling mechanism for Tem1 differs from the canonical Ras-like GTPase signaling paradigm and is likely relevant to other localization-based signaling scenarios.