Type V Myosin focuses the polarisome and shapes the tip of yeast cells

The polarisome is a cortical proteinaceous micro-compartment that organizes the growth of actin filaments and the fusion of secretory vesicles in yeasts and filamentous fungi. Polarisomes are compact, spot-like structures at the growing tips of their respective cells. The molecular forces that control form and size of this micro-compartment are not known. Here we identify a complex between the polarisome subunit Pea2 and the type V Myosin Myo2 that anchors Myo2 at the cortex of yeast cells. We discovered a point mutation in the cargo-binding domain of Myo2 that impairs the interaction with Pea2 and consequently the formation and focused localization of the polarisome. Cells carrying this mutation grow round instead of elongated buds. Further experiments and biophysical modeling suggest that polarisome nanoparticles use multiple copies of Myo2 and an actin filament polymerizing activity to drive the assembly of the polarisome and sustain its compact shape.