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 (PubMed:34715025). Contributes to normal sperm motility (By similarity). Found in the abaxial (convex) surface of outer dense fibers in sperm flagella. Expressed in trachea multiciliated cells. Belongs to the tektin family.