Model of augmin-mediated branching MT nucleation and its role in cytokinesis. (A) Based on negative stain EM of the augmin complex, we propose that the augmin complex contains a rigid ∼30-nm-long stem that binds MTs at one end and an ∼15-nm flexible splayed end that recruits γ-TuRC through Dgt6 and possibly other interfaces. MTBR, MT binding region in Hice 1/HAUS8. (B) Key to molecular schematics in the figure. Note the overlaid “Y” on the augmin complex schematic, which is drawn based on negative stain EM of the complex. (C) A flexible hinge region in the augmin complex and/or flexibility in the splayed Y-end of augmin may allow a daughter MT to sample a broad range of possible branch angles relative to its mother. The branch angle is impacted by the local cellular environment such that “unbridled” daughter MTs with fewer spatial constraints (e.g., astral MTs in anaphase) would have larger branch angles than daughters nucleated in the spatially constrained environment of the spindle and midzone MT array. (D) Branched MT arrays may amplify RhoA activation by generating more MT plus ends that are capable of recruiting cortical ECT2 via direct interaction with plus end–bound (polo-phosphorylated) centralspindlin.