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). Contributes to normal sperm motility (By similarity). Found in the abaxial (convex) surface of outer dense fibers in sperm flagella. Belongs to the tektin family.