Heteroallelic combination of anillin mutants reveals cytoskeletal uncoupling during cytokinesis.

Cytokinetic contractile rings close, disassemble and transition to midbody rings through poorly understood mechanisms. We previously showed that this transition in Drosophila S2 cells requires both Citron kinase (Sticky) and septins, acting on the N- and C-terminus, respectively, of the scaffold protein, Anillin. To better understand the coordination of these proteins, we performed deplete-and-rescue experiments using Anillin mutants defective in Sticky interaction or septin recruitment. Expressing each mutant individually as two copies tagged with GFP and mCherry revealed comparable behaviors of the tags and characteristic cytokinesis failure phenotypes. However, trans-heterozygous combination of both Anillin mutants rescued the cytokinesis defects, with both mutants clearly exhibiting different localization patterns. Thus, the essential N- and C-terminus-mediated roles of Anillin can be satisfied by different molecules of Anillin that cannot bind to Sticky and to septins simultaneously. We conclude that Anillin organizes distinct cytoskeletal elements that are uncoupled from one another and that their physical separation drives the transition from contractile ring to midbody ring.