Condensation of the fusion focus by the intrinsically disordered region of the formin Fus1 is essential for cell-cell fusion

Secretory vesicle clusters transported on actin filaments by myosin V motors for local secretion underlie various cellular processes, such as neurotransmitter release at neuronal synapses,1 hyphal steering in fila-mentous fungi,2,3 and local cell wall digestion preceding the fusion of yeast gametes.4 During fission yeast Schizosaccharomyces pombe gamete fusion, the actin fusion focus assembled by the formin Fus1 concen-trates secretory vesicles carrying cell wall digestive enzymes.5-7 The position and coalescence of the vesicle focus are controlled by local signaling and actin-binding proteins to prevent inappropriate cell wall digestion that would cause lysis,6,8-10 but the mechanisms of focusing have been elusive. Here, we show that the reg-ulatory N terminus of Fus1 contains an intrinsically disordered region (IDR) that mediates Fus1 condensation in vivo and forms dense assemblies that exclude ribosomes. Fus1 lacking its IDR fails to concentrate in a tight focus and causes cell lysis during attempted cell fusion. Remarkably, the replacement of Fus1 IDR with a het-erologous low-complexity region that forms molecular condensates fully restores Fus1 focusing and func-tion. By contrast, the replacement of Fus1 IDR with a domain that forms more stable oligomers restores focusing but poorly supports cell fusion, suggesting that condensation is tuned to yield a selectively perme-able structure. We propose that condensation of actin structures by an IDR may be a general mechanism for actin network organization and the selective local concentration of secretory vesicles.

Automated summary

Secretory vesicle clusters transported on actin filaments by myosin V motors play important roles in local secretion, and their focusing mechanism involves condensation mediated by an intrinsically disordered region (IDR) and selective permeability. This condensation mechanism may be a general mechanism for actin network organization and selective local concentration of secretory vesicles.