A systematic screen for morphological abnormalities during fission yeast sexual reproduction identifies a mechanism of actin aster formation for cell fusion

In non-motile fungi,sexual reproduction relies on strong morphogenetic changes in response to pheromone signaling. we report here on a systematic screen for morphological abnormalities of the mating process in fission yeast schizosaccharomyces pombe. we derived a homothallic (self-fertile) collection of viable deletions,which,upon visual screening,revealed a plethora of phenotypes affecting all stages of the mating process,including cell polarization,cell fusion and sporulation. cell fusion relies on the formation of the fusion focus,an aster-like f-actin structure that is marked by strong local accumulation of the myosin v myo52,which concentrates secretion at the fusion site. a secondary screen for fusion-defective mutants identified the myosin v myo51-associated coiled-coil proteins rng8 and rng9 as critical for the coalescence of the fusion focus. indeed,rng8δ and rng9δ mutant cells exhibit multiple stable dots at the cell-cell contact site,instead of the single focus observed in wildtype. rng8 and rng9 accumulate on the fusion focus,dependent on myo51 and tropomyosin cdc8. a tropomyosin mutant allele,which compromises rng8/9 localization but not actin binding,similarly leads to multiple stable dots instead of a single focus. by contrast,myo51 deletion does not strongly affect fusion focus coalescence. we propose that focusing of the actin filaments in the fusion aster primarily relies on rng8/9-dependent cross-linking of tropomyosin-actin filaments. © 2017 dudin et al.