Microtubule inner protein (MIP) part of the dynein-decorated doublet microtubules (DMTs) in cilia and flagellar axoneme. Forms filamentous polymers in the walls of ciliary and flagellar microtubules (By similarity). Required for normal sperm mobility (By similarity). In the sperm flagellum, localizes to the periaxonemal region where it associates with the mitochondrial sheath and outer dense fibers (By similarity). Not detected in the central axonemal region of the flagellum (By similarity). Associates with the acrosome membrane in the equatorial segment of the sperm head (By similarity). Also detected just below the plasma membrane in the post-acrosomal region where it might localize to the postacrosomal dense lamina (By similarity). However, other studies report little or no expression in the postacrosomal region (By similarity). Translocates from the postacrosomal region to the equatorial segment after sperm activation (By similarity). Retained in the postacromal region, but not the equatorial segment, following the acrosome reaction (By similarity). Some studies report strong expression in the anterior cap region (By similarity). However, other studies report little or no expression in the acrosomal cap (By similarity). N- and O-glycosylated. May be proteolytically processed during the epididymal transit of spermatozoa. Belongs to the tektin family.