Microtubule inner protein (MIP) part of the dynein-decorated doublet microtubules (DMTs) in cilia and flagellar axoneme (PubMed:36191189). Forms filamentous polymers in the walls of ciliary and flagellar microtubules (By similarity). Required for normal sperm mobility (By similarity). {ECO:0000250|UniProtKB:A6H782, ECO:0000250|UniProtKB:Q6X6Z7, ECO:0000269|PubMed:36191189}. This is the function of Ensembl gene identifier ENSG00000125409 (TEKT3, tektin 3).