Microtubule minus-end aster organization is driven by processive HSET-tubulin clusters
Higher-order structures of the microtubule (MT) cytoskeleton are comprised of two architectures: bundles and asters. Although both architectures are critical for cellular function, the molecular pathways that drive aster formation are poorly understood. Here, we study aster formation by human minus-end directed kinesin-14 (HSET/KIFC1). We show that HSET is incapable of forming asters from pre-formed, non-growing MTs, but rapidly forms MT asters in the presence of soluble tubulin. HSET binds soluble (non-polymer) tubulin via its N-terminal tail domain to form heterogeneous HSET-tubulin \"clusters\" containing multiple motors. Cluster formation induces motor processivity and rescues the formation of asters from non-growing MTs. We then show that excess soluble tubulin stimulates aster formation in HeLa cells overexpressing HSET during mitosis. We propose a model where HSET can toggle between MT bundle and aster formation in a manner governed by the availability of soluble tubulin.