Differential Regulation of Single Microtubules and Cross-Linked Bundles by a Minimal Three-Protein Module

A remarkable feature of the microtubule cytoskeleton is co-existence of distinct populations having different dynamic properties. A prominent example is the anaphase spindle, where stable antiparallel bundles exist alongside dynamic microtubules and provide spatial cues for cytokinesis. How are dynamics of spatially proximal arrays differentially regulated? We reconstitute a minimal system of three midzone proteins: the microtubule-cross-linker PRC1, and its interactors CLASP1 and Kif4A, proteins that promote and suppress microtubule elongation, respectively. We find their collective activity promotes elongation of single microtubules, while simultaneously stalling polymerization of cross-linked bundles. This striking differentiation arises from (i) higher activity of CLASP1 despite lower microtubule occupancy than Kif4A, (ii) amplification of Kif4A activity on microtubule bundles through stronger PRC1-affinity. In contrast to canonical mechanisms where antagonistic regulators set microtubule lengths of single arrays, our findings illuminate design principles by which their collective activity instead creates microenvironments comprised of arrays with distinct dynamic properties.