Microtubule minus-end aster organization is driven by processive HSET-tubulin clusters

Area of Science:

• Cell Biology
• Cytoskeletal Dynamics
• Molecular Motors

Background:

• Microtubule (MT) cytoskeleton exhibits higher-order structures like bundles and asters, crucial for cellular functions.
• Molecular mechanisms governing MT aster formation remain largely unelucidated.

Purpose of the Study:

• Investigate the role of human minus-end-directed kinesin-14 (HSET/KIFC1) in MT aster formation.
• Determine the influence of soluble tubulin on HSET-mediated aster assembly.

Main Methods:

• In vitro assays using preformed, non-growing MTs and soluble tubulin.
• Biochemical analysis of HSET-tubulin interactions and cluster formation.
• Cellular experiments in HeLa cells overexpressing HSET during mitosis.

Main Results:

• HSET alone cannot form asters from non-growing MTs but rapidly forms asters with soluble tubulin.
• HSET binds soluble tubulin via its N-terminal tail, forming heterogeneous HSET-tubulin clusters.
• Cluster formation enhances HSET motor processivity, rescuing aster formation from non-growing MTs.
• Excess soluble tubulin promotes aster formation in mitotic HeLa cells overexpressing HSET.

Conclusions:

• Soluble tubulin availability dictates HSET's ability to form MT asters versus bundles.
• HSET acts as a switch between MT bundle and aster formation based on tubulin concentration.
• A model is proposed where HSET-tubulin interactions regulate cytoskeletal architecture.