A Non-canonical BRCT-Phosphopeptide Recognition Mechanism Underlies RhoA Activation in Cytokinesis

Cytokinesis partitions the cell contents to complete mitosis. During cytokinesis, polo-like kinase 1 (PLK1) activates the small GTPase RhoA to assemble a contractile actomyosin ring. PLK1 is proposed to pattern RhoA activation by creating a docking site on the central spindle that concentrates the RhoA guanine nucleotide exchange factor ECT2. However, ECT2 targeting to the central spindle is dispensable for cytokinesis, indicating that how PLK1 controls RhoA activation remains unresolved. To address this question, we employed an unbiased approach targeting ∼100 predicted PLK1 sites in two RhoA regulators: ECT2 and the centralspindlin complex, composed of CYK4 and kinesin-6. This comprehensive approach suggested that the only functionally critical PLK1 target sites are in a single cluster in the CYK4 N terminus. Phosphorylation of this cluster promoted direct interaction of CYK4 with the BRCT repeat module of ECT2. However, mutational analysis in vitro and in vivo led to the surprising finding that the interaction was independent of the conserved "canonical" residues in ECT2's BRCT repeat module that, based on structurally characterized BRCT-phosphopeptide interactions, were presumed critical for binding. Instead, we show that the ECT2 BRCT module binds phosphorylated CYK4 via a distinct conserved basic surface. Basic surface mutations mimic the effects on cytokinesis of loss of CYK4 cluster phosphorylation or inhibition of PLK1 activity. Together with evidence for ECT2 autoinhibition limiting interaction with CYK4 in the cytoplasm, these results suggest that a spatial gradient of phosphorylated CYK4 around the central spindle patterns RhoA activation by interacting with ECT2 on the adjacent plasma membrane.