Cytoskeleton: Microtubule tip tracker

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The microtubule-associated protein XMAP215 functions as a processive polymerase that promotes microtubule growth by facilitating the addition of multiple rounds of tubulin to the growing microtubule end.

The microtubule-associated protein XMAP215 promotes microtubule growth, but how this occurs has been unclear. To address this question, Gary Brouhard et al. developed a single-molecule fluorescence microscopy assay to visualize microtubule growth. As expected, growth occurs from the plus end of the microtubule, and XMAP215 associates both with growing and shrinking microtubule plus ends.

The authors followed individual XMAP215 molecules during microtubule polymerization and measured a mean residence time of almost 4 seconds, during which time the microtubule grew by $sim$ 0.2 µm, which corresponded to the addition of $sim$ 25 tubulin dimers. XMAP215 moves with the growing plus end, and the mean displacement is similar to the measured growth rate of microtubules. Therefore, single XMAP215 molecules appear to 'tip-track' the plus end of the growing microtubule.

But how does XMAP215 promote the addition of so many tubulin dimers during its interaction with the microtubule end? Using negative-stain electron microscopy, the authors observed a complex resembling a monomeric XMAP215 rod that closes up around a tubulin dimer. Size-exclusion chromatography confirmed that XMAP215 and tubulin dimers form a 1:1 complex. Using total internal reflection fluorescence microscopy, Brouhard and colleagues showed the same 1:1 stoichiometry of XMAP215 and tubulin on the microtubule lattice. Together, these findings contradict the 'tubulin shuttle' model, according to which multiple tubulin dimers are brought to the microtubule end.

Instead, the authors propose that XMAP215 functions as a processive polymerase, whereby XMAP215 promotes multiple rounds of tubulin addition to the growing microtubule end. GTP hydrolysis is not required for XMAP215 to promote microtubule growth, which led to the suggestion that XMAP215 stabilizes a weakly bound tubulin dimer that subsequently becomes incorporated in the microtubule lattice. In agreement with this model, XMAP215 aids microtubule shrinkage in the absence of free tubulin by pulling tubulin dimers from the lattice.

This 'tip-track' mechanism of XMAP215 is similar to that of formins, which catalyse the addition of multiple actin monomers to the growing end of actin filaments. So, tip-tracking by a processive polymerase may be a common mechanism for stimulating the growth of cytoskeletal filaments.

Arianne Heinrichs

References

Brouhard, G. J. et al. XMAP215 is a processive microtubule polymerase. Cell 132, 79–88 (2008) | Article | PubMed |