Calcium sensitivity of α-actinin is required for equatorial actin assembly during cytokinesis

The actin cross-linking protein, α-actinin, plays a crucial role in mediating furrow ingression during cytokinesis. However, the mechanism by which its dynamics are regulated during this process is poorly understood. Here we have investigated the role of calcium sensitivity of α-actinin in the regulation of its dynamics by generating a functional calcium-insensitive mutant (EFM). GFP-tagged EFM (EFM-GFP) localized to the equatorial regions during cell division. However, the maximal equatorial accumulation of EFM-GFP was significantly smaller in comparison to α-actinin-GFP when it was expressed in normal cells and cells depleted of endogenous α-actinin. No apparent defects in cytokinesis were observed in these cells. However, F-actin levels at the equator were significantly reduced in cells expressing EFM-GFP as compared with α-actinin-GFP at furrow initiation but were recovered during furrow ingression. These results suggest that calcium sensitivity of α-actinin is required for its equatorial accumulation that is crucial for the initial equatorial actin assembly but is dispensable for cytokinesis. Equatorial RhoA localization was not affected by EFM-GFP overexpression, suggesting that equatorial actin assembly is predominantly driven by the RhoA-dependent mechanism. Our observations shed new light on the role and regulation of the accumulation of pre-existing actin filaments in equatorial actin assembly during cytokinesis.