CLASP's dictate the tubulin off-rate to regulate microtubule dynamics.

Abstract

CLASPs constitute a family of conserved TOG-domain proteins that regulate microtubules in many fundamental cellular processes. CLASPs stabilize dynamic microtubules by suppressing catastrophe and promoting rescue, the switch-like transitions between growth and shrinkage. However, the molecular mechanisms underlying CLASPs’ activity are not fully understood. Here, we report that human CLASP1 depolymerizes stable microtubules in the absence of tubulin, an activity previously reported for another TOG-domain protein XMAP215. Surprisingly, we find that, unlike XMAP215, CLASP activity is nucleotide-dependent, depolymerizing stable microtubules in the presence of GTP and GDP, but not in the presence of GMPCPP nor in the absence of nucleotide. We demonstrate that other human CLASP family members (CLASP2α and CLASP2γ) also exhibit nucleotide-dependent depolymerase activity on stable microtubules, as does a TOG2-domain construct, the minimal unit required for regulating microtubule dynamics. By investigating CLASP's effects on microtubules grown with GTP, GMPCPP, or a mixture of the two nucleotides, we find that CLASP1 dictates the rate of microtubule depolymerization regardless of microtubule substrate. Therefore, we conclude that CLASP1 regulates the tubulin-off rate at microtubule ends. The unanticipated finding that CLASPs dictate the tubulin-off rate in a nucleotide-dependent manner underlies CLASPs activity on dynamic microtubules and provides critical mechanistic insights into an important family of microtubule regulatory proteins.