A Trimer Consisting of the Tubulin-specific Chaperone TBCD, Regulatory GTPase ARL2, and {beta}-tubulin is Required for Maintaining the Microtubule Network [Signal Transduction]

Microtubule dynamics involves the polymerization and depolymerization of tubulin dimers and is an essential and highly regulated process required for cell viability, architecture, and division. The regulation of the microtubule network also depends upon the maintenance of a pool of αβ-tubulin heterodimers. These dimers are the end result a complex folding and assembly events, requiring the TriC/CCT chaperonin and five tubulin-specific chaperones, TBCA-E. However, models of the actions of these chaperones are incomplete or inconsistent. We previously purified TBCD from bovine tissues and showed that it tightly binds the small GTPase ARL2, yet appears to be inactive. Here, in an effort to identify the functional form of TBCD and using non-denaturing gels and immunoblotting, we analyzed lysates from a number of mouse tissues and cell lines to identify the quaternary state(s) of TBCD and ARL2. We found that both proteins co-migrated in native gels in a complex of ~200 kDa that also contained β-tubulin. Using human embryonic kidney cells enabled the purification of the TBCD/ARL2/β-tubulin trimer found in cell and tissue lysates, as well as two other novel TBCD complexes. Characterization of ARL2 point mutants that disrupt binding to TBCD suggested that the ARL2-TBCD interaction is critical for proper maintenance of microtubule densities in cells. We conclude that the TBCD/ARL2/β-tubulin trimer represents a functional complex whose activity is fundamental to microtubule dynamics.

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