Cryo-electron tomography suggests tubulin chaperones form a subset of microtubule lumenal particles with a role in maintaining neuronal microtubules

The functional architecture of the long-lived neuronal microtubule (MT) cytoskeleton is maintained by various MT-associated proteins (MAPs), most of which are known to bind to the MT outer surface. However, electron microscopy (EM) has long ago revealed the presence of particles inside the lumens of neuronal MTs, of yet unknown identity and function. Here, we use cryogenic electron tomography (cryo-ET) to analyze the three-dimensional (3D) structures and organizations of MT lumenal particles in primary hippocampal neurons, human induced pluripotent stem cell-derived neurons and pluripotent P19 cells. We obtain in-cell 3D maps of several lumenal particles from the respective cells and detect structural features that are common to all cell-types, underscoring their potential overarching functions. Mass spectrometry-based proteomics combined with structural modeling suggests a subset of lumenal particles could be tubulin-binding cofactors (TBCs) bound to tubulin monomers. A different subset of smaller particles, which remains unidentified, exhibits densities that bridge across the MT protofilaments. We show that increased lumenal particle concentration within MTs is concomitant with neuronal differentiation and correlates with higher MT curvatures. Enrichment of lumenal particles around MT lattice defects and at freshly polymerized MT open-ends suggest a MT protective role. Together with the identified structural resemblance of a subset of particles to TBCs, these results hint at a role in local tubulin proteostasis for the maintenance of long-lived neuronal MTs.