CSPP1 stabilizes growing microtubule ends and damaged lattices from the luminal side

Abstract

Microtubules are dynamic cytoskeletal polymers, and their organization and stability are tightly regulated by numerous cellular factors. While regulatory proteins controlling formation of interphase microtubule arrays and mitotic spindles have been extensively studied, the biochemical mechanisms responsible for generating stable microtubule cores of centrioles and cilia are poorly understood. Here, we used in vitro reconstitution assays to investigate microtubule-stabilizing properties of CSPP1, a centrosome and cilia-associated protein mutated in the neurodevelopmental ciliopathy Joubert syndrome. We found that CSPP1 preferentially binds to polymerizing microtubule ends that grow slowly or undergo growth perturbations and, in this way, resembles microtubule-stabilizing compounds such as taxanes. Fluorescence microscopy and cryo-electron tomography showed that CSPP1 is deposited in the microtubule lumen and inhibits microtubule growth and shortening through two separate domains. CSPP1 also specifically recognizes and stabilizes damaged microtubule lattices. These data help to explain how CSPP1 regulates elongation and stability of ciliary axonemes and other microtubule-based structures.

Lumenal components of cytoplasmic microtubules

“…At this point, it is perhaps worth returning to studies of axonemal microtubules where structural and functional understanding is more advanced [5][6][7][8][9][10][11][12][13][14][15][16]. Numerous MIPs, both globular and filamentous, form a periodic mesh lining the inner wall of the microtubule, some of which protrude into the microtubule wall itself, to stabilise the microtubule in cilia and sperm flagella.…”

“…In studies of axonemal microtubules, a plethora of microtubule inner proteins (MIPs) have been identified which associate with the inner surface of the microtubule wall, and generally appear to be important for microtubule stability and structure. These studies have been reviewed elsewhere, with some very rapid recent progress, and so will not be considered in detail in this review [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19]. Here, we consider the series of historic and recent studies that have focused on the lumenal contents of cytoplasmic microtubules which are less well understood.…”