CLASP mediates microtubule repair by promoting tubulin incorporation into damaged lattices

Abstract

Microtubule network plays a key role in cell division, motility and intracellular trafficking. Microtubule lattices are generally regarded as stable structures that undergo turnover through dynamic instability of their ends [1]. However, recent evidence suggests that microtubules also exchange tubulin dimers at the sites of lattice defects, which can either be induced by mechanical stress or occur spontaneously during polymerization [2-4]. Tubulin incorporation can restore microtubule integrity; moreover, \"islands\" of freshly incorporated GTP-tubulin can inhibit microtubule disassembly and promote rescues [3-7]. Microtubule repair occurs in vitro in the presence of tubulin alone [2-4, 8]. However, in cells, it is likely to be regulated by specific factors, the nature of which is currently unknown. CLASP is an interesting candidate for microtubule repair, because it induces microtubule nucleation, stimulates rescue and suppresses catastrophes by stabilizing incomplete growing plus ends with lagging protofilaments and promoting their conversion into complete ones [9-16]. Here, we used in vitro reconstitution assays combined with laser microsurgery and microfluidics to show that CLASP2 indeed stimulates microtubule lattice repair. CLASP2 promoted tubulin incorporation into damaged lattice sites thereby restoring microtubule integrity. Furthermore, it induced the formation of complete tubes from partial protofilament assemblies and inhibited microtubule softening caused by hydrodynamic flow-induced bending. A single CLASP2 domain, TOG2, which suppresses catastrophes when tethered to microtubules, was sufficient to stimulate microtubule repair, indicating that catastrophe suppression and lattice repair are mechanistically similar. Our results suggest that the cellular machinery controlling microtubule nucleation and growth can also help to maintain microtubule integrity.