The XMAP215 family drives microtubule polymerization using a structurally diverse TOG array.

Area of Science:

• Molecular Biology
• Structural Biology
• Cell Biology

Background:

• XMAP215 proteins are key microtubule (MT) polymerases, essential for cell division.
• These proteins feature an array of tumor overexpressed gene (TOG) domains that bind tubulin.
• The structural and functional equivalence of these TOG domains remains largely uncharacterized.

Purpose of the Study:

• To determine the structural architecture of TOG domains within the XMAP215 family.
• To investigate the tubulin-binding affinities and polymerization effects of individual TOG domains.
• To elucidate the functional importance of each TOG domain in MT polymerization.

Main Methods:

• X-ray crystallography to determine the structures of TOG4 domains from Drosophila Msps and human ch-TOG.
• In vitro biochemical assays to measure tubulin-binding affinities and MT polymerization rates.
• Functional studies in Drosophila S2 cells using gene depletion and domain-specific mutations.

Main Results:

• TOG4 domains exhibit distinct structures compared to TOG1 and TOG2 domains, featuring a conserved bend.
• Differential tubulin-binding affinities and MT polymerization effects were observed across the TOG array.
• Mutations in individual TOG domain tubulin-binding sites significantly impaired MT polymerization rescue in cells.

Conclusions:

• The TOG array in XMAP215 proteins is structurally diverse but positionally conserved.
• Each TOG domain plays a critical, non-redundant role in regulating MT polymerization.
• The unique architecture of TOG domains contributes to their specific functions in MT dynamics.