Preference of CAMSAP3 for microtubules with expanded lattice contributes to stabilization of the minus end

CAMSAPs are proteins that characteristically show microtubule minus-end-specific localization, decoration and stabilization. Although the mechanism for minus-end recognition via their C-terminal CKK domain has been well described in recent structural studies, it is unclear how CAMSAPs stabilize microtubules and why the stabilizing effect varies among the three family members, CAMSAP1, 2 and 3. We conducted several binding assays under fluorescence microscopy and revealed that D2, the helical region of CAMSAP3 next to the CKK domain, specifically binds to microtubules with the expanded lattice, such as GMPCPP-polymerized, taxol-stabilized and kinesin-pretreated microtubules. To investigate the relationship between this preference and the stabilization effect of CAMSAP3, we precisely measured individual microtubule lengths and found that D2-binding expanded the microtubule lattice by ∼3%. Consistent with the notion that the expanded lattice is a common feature of stable microtubules, the presence of D2 slowed the microtubule depolymerization rate to approximately 1/20, suggesting that the D2-triggered lattice expansion stabilizes microtubules. Combining these results, we propose that CAMSAP3 stabilizes microtubules in two steps: first, by recruitment to the microtubule minus end via the CKK domain, and then by lattice expansion upon D2-binding to stabilize the microtubule and accelerate the recruitment of other CAMSAP3 molecules. Since only CAMSAP3 has D2 and the highest microtubule stabilizing effect among mammalian CAMSAPs, our model also explains the molecular basis for the functional diversity of CAMSAP family members.