Domain-autonomous MKLP1 modules divergently regulate mitotic fidelity and polyploidy in a holocentric system.

While the kinesin MKLP1 is a well-established regulator of cytokinesis, whether its individual domains possess autonomous functions has remained unexplored. Here, using the holocentric Bombyx mori BmN4 system, we show that isolated head, stalk, and tail domains of MKLP1 exhibit distinct subcellular localization patterns and exert divergent control over mitosis. These domains differentially regulate cell-cycle progression, with the head and stalk inducing G2/M arrest, whereas the tail, and to a lesser extent the head, promotes polyploidization. Despite these divergent outcomes, all three domains converge on a shared pathological activity, aberrantly recruiting γ-tubulin, disrupting spindle organization, and compromising chromosome segregation. The domains also exert opposing effects on EdU incorporation, with the head suppressing EdU labeling and the stalk and tail enhancing it. Together, our findings redefine MKLP1 as a composite platform of regulatory modules, whose uncoupling reveals a principle of kinesin organization with broad implications for mitotic robustness and genomic stability.