Structures of Drosophila Msps TOG4 and human ch-TOG TOG4 are presented. TOG4 departs from the other TOG structures and predicts novel α-tubulin engagement. Whereas TOG domains across the array have different tubulin-binding properties, cellular studies show that a fully functional TOG array is required for microtubule polymerase activity.

Background: XMAP215 family members contain arrayed TOG domains that promote microtubule polymerization. Results: TOG3 has a unique architecture that positions conserved residues in a TOG1-and TOG2-like tubulin-binding mode. Conclusion: TOG1, TOG2, and TOG3 have similar architectures and predicted tubulin-binding modes that contrast with TOG4. Significance: XMAP215 family members use a structurally diverse, polarized TOG domain array to promote microtubule polymerization.

The yeast microtubule polymerase Stu2’s C-terminal domain is a 15-nm parallel, homodimeric coiled coil with two spatially distinct conserved regions. Determinants in these conserved regions optimally position Stu2 on the mitotic spindle to drive proper spindle structure and dynamics.

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