Microtubule lattice spacing governs cohesive envelope formation of tau family proteins.

Tau is an intrinsically disordered microtubule-associated protein (MAP) implicated in neurodegenerative disease. On microtubules, tau molecules segregate into two kinetically distinct phases, consisting of either independently diffusing molecules or interacting molecules that form cohesive 'envelopes' around microtubules. Envelopes differentially regulate lattice accessibility for other MAPs, but the mechanism of envelope formation remains unclear. Here we find that tau envelopes form cooperatively, locally altering the spacing of tubulin dimers within the microtubule lattice. Envelope formation compacted the underlying lattice, whereas lattice extension induced tau envelope disassembly. Investigating other members of the tau family, we find that MAP2 similarly forms envelopes governed by lattice spacing, whereas MAP4 cannot. Envelopes differentially biased motor protein movement, suggesting that tau family members could spatially divide the microtubule surface into functionally distinct regions. We conclude that the interdependent allostery between lattice spacing and cooperative envelope formation provides the molecular basis for spatial regulation of microtubule-based processes by tau and MAP2.

A highly conserved 310 helix within the kinesin motor domain is critical for kinesin function and human health.

KIF1A is a critical cargo transport motor within neurons. More than 100 known mutations result in KIF1A-associated neurological disorder (KAND), a degenerative condition for which there is no cure. A missense mutation, P305L, was identified in children diagnosed with KAND, but the molecular basis for the disease is unknown. We find that this conserved residue is part of an unusual 310 helix immediately adjacent to the family-specific K-loop, which facilitates a high microtubule-association rate. We find that the mutation negatively affects several biophysical parameters of the motor. However, the microtubule-association rate of the motor is most markedly affected, revealing that the presence of an intact K-loop is not sufficient for its function. We hypothesize that the 310 helix facilitates a specific K-loop conformation that is critical for its function. We find that the function of this proline is conserved in kinesin-1, revealing a fundamental principle of the kinesin motor mechanism.