Chakraborty S, Martinez-Sanchez A, Beck F, Toro-Nahuelpan M, Hwang IY, Noh KM, Baumeister W, Mahamid J
Functional architecture of the neuronal microtubule (MT) cytoskeleton is maintained by various MT-associated proteins (MAPs), most of which bind to the MT outer surface. Yet, electron microscopy (EM) has long revealed hitherto unknown electron-dense particles inside the lumens of neuronal MTs. Here, we use cryogenic electron tomography (cryo-ET) to analyze the native three-dimensional (3D) structures and organization of MT lumenal particles inside vitrified rodent primary neurons, pluripotent P19 cells and human induced pluripotent stem cell-derived neurons. We obtain 3D maps of several lumenal particles at molecular resolution that periodically decorate neuronal MTs. We show that increased lumenal particle localization is concomitant with neuronal differentiation and correlates with higher MT curvatures. Lumenal particles binding topology in MTs, their structural resemblance to tubulin binding cofactors (TBCs), enrichment around MT lattice defects and at plus-ends indicated their potential role in tubulin proteostasis for the maintenance of neuronal MTs.