Cilium- and flagellum-specific protein that plays a role in axonemal structure organization and motility. Microtubule inner protein (MIP) part of the dynein-decorated doublet microtubules (DMTs) in cilia axoneme, which is required for motile cilia beating. Involved in the regulation of the size and morphology of cilia. Required for axonemal microtubules polyglutamylation