Drosophila melanogaster Mini Spindles TOG3 Utilizes Unique Structural Elements to Promote Domain Stability and Maintain a TOG1- and TOG2-like Tubulin-binding Surface

Amy E Howard,Jaime C Fox,Kevin C Slep

doi:10.1074/jbc.m114.633826

Amy E Howard, Jaime C Fox + Show 1 more

Open Access

https://doi.org/10.1074/jbc.m114.633826

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Abstract

Microtubule-associated proteins regulate microtubule (MT) dynamics spatially and temporally, which is essential for proper formation of the bipolar mitotic spindle. The XMAP215 family is comprised of conserved microtubule-associated proteins that use an array of tubulin-binding tumor overexpressed gene (TOG) domains, consisting of six (A-F) Huntingtin, elongation factor 3, protein phosphatase 2A, target of rapamycin (HEAT) repeats, to robustly increase MT plus-end polymerization rates. Recent work showed that TOG domains have differentially conserved architectures across the array, with implications for position-dependent TOG domain tubulin binding activities and function within the XMAP215 MT polymerization mechanism. Although TOG domains 1, 2, and 4 are well described, structural and mechanistic information characterizing TOG domains 3 and 5 is outstanding. Here, we present the structure and characterization of Drosophila melanogaster Mini spindles (Msps) TOG3. Msps TOG3 has two unique features as follows: the first is a C-terminal tail that stabilizes the ultimate four HEAT repeats (HRs), and the second is a unique architecture in HR B. Structural alignments of TOG3 with other TOG domain structures show that the architecture of TOG3 is most similar to TOG domains 1 and 2 and diverges from TOG4. Docking TOG3 onto recently solved Stu2 TOG1· and TOG2·tubulin complex structures suggests that TOG3 uses similarly conserved tubulin-binding intra-HEAT loop residues to engage α- and β-tubulin. This indicates that TOG3 has maintained a TOG1- and TOG2-like TOG-tubulin binding mode despite structural divergence. The similarity of TOG domains 1-3 and the divergence of TOG4 suggest that a TOG domain array with polarized structural diversity may play a key mechanistic role in XMAP215-dependent MT polymerization activity.

Highlights

XMAP215 family members contain arrayed tumor overexpressed gene (TOG) domains that promote microtubule polymerization
Our findings suggest that TOG3 binds ␣␤-tubulin in a mode similar to TOG1 and TOG2 but different from TOG4, highlighting the polarized nature of architecturally distinct TOG domains in the pentameric XMAP215 family TOG domain array, which is collectively used to promote the rate of MT polymerization
TOG domain structures have been solved across multiple XMAP215 family members including Stu2 TOG1 and TOG2, ZYG-9 TOG3, Mini spindles (Msps) TOG2 and TOG4, and ch-TOG TOG4 (Fig. 1A, colored boxes) (16, 38 – 41)

Summary

Background

XMAP215 family members contain arrayed TOG domains that promote microtubule polymerization. Two distinct questions arise from the determination that TOG domains have differential architectures and abilities to promote MT plus-end polymerization; the first concerns whether TOG domains in the pentameric array have a similar or distinct function, and the second concerns whether all the TOG domains bind free tubulin or a subset selectively binds MT lattice-incorporated tubulin The latter suggests that XMAP215 TOG domain arrays are composed of position-specific TOG domain architectures that recognize different tubulin structural states, which may play a key mechanistic role in XMAP215-mediated MT polymerization. Our findings suggest that TOG3 binds ␣␤-tubulin in a mode similar to TOG1 and TOG2 but different from TOG4, highlighting the polarized nature of architecturally distinct TOG domains in the pentameric XMAP215 family TOG domain array, which is collectively used to promote the rate of MT polymerization

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