The F-actin bundler alpha-actinin Ain1 is tailored for ring assembly and constriction during cytokinesis in fission yeast

The actomyosin contractile ring is a network of crosslinked actin filaments that facilitates cytokinesis in dividing cells. Contractile ring formation has been well characterized in Schizosaccharomyces pombe, where the crosslinking protein α-actinin SpAin1 bundles the actin filament network. However, the specific biochemical properties of SpAin1 and whether they are tailored for cytokinesis are not known. Therefore, we purified SpAin1 and quantified its ability to dynamically bind and bundle actin filaments in vitro using a combination of bulk sedimentation assays and direct visualization by two-color TIRF microscopy. We found that while SpAin1 bundles actin filaments of mixed polarity like other α-actinins, SpAin1 has lower bundling activity and is more dynamic than human α-actinin HsACTN4. In order to determine if dynamic bundling is important for cytokinesis in fission yeast, we created the less dynamic bundling mutant SpAin1(R216E). We found that dynamic bundling is critical for cytokinesis, as cells expressing SpAin1(R216E) display disorganized ring material and delays in both ring formation and constriction. Furthermore, computer simulations of initial actin filament elongation and alignment revealed that an intermediate level of crosslinking best facilitates filament alignment. Together, our results demonstrate that dynamic bundling by SpAin1 is important for proper contractile ring formation and constriction.