SPACA9 is a lumenal protein of human ciliary singlet and doublet microtubules.

Gui M, Croft JT, Zabeo D, Acharya V, Kollman JM, Burgoyne T, Höög JL, Brown A

Proc. Natl. Acad. Sci. U.S.A. 119 (41) e2207605119 [2022-10-11; online 2022-10-03]

The cilium-centrosome complex contains triplet, doublet, and singlet microtubules. The lumenal surfaces of each microtubule within this diverse array are decorated by microtubule inner proteins (MIPs). Here, we used single-particle cryo-electron microscopy methods to build atomic models of two types of human ciliary microtubule: the doublet microtubules of multiciliated respiratory cells and the distal singlet microtubules of monoflagellated human spermatozoa. We discover that SPACA9 is a polyspecific MIP capable of binding both microtubule types. SPACA9 forms intralumenal striations in the B tubule of respiratory doublet microtubules and noncontinuous spirals in sperm singlet microtubules. By acquiring new and reanalyzing previous cryo-electron tomography data, we show that SPACA9-like intralumenal striations are common features of different microtubule types in animal cilia. Our structures provide detailed references to help rationalize ciliopathy-causing mutations and position cryo-EM as a tool for the analysis of samples obtained directly from ciliopathy patients.

Cryo-EM [Service]

PubMed 36191189

DOI 10.1073/pnas.2207605119

Crossref 10.1073/pnas.2207605119

pmc: PMC9564825

Publications 8.1.1