Microtubule inner protein (MIP) part of the dynein-decorated doublet microtubules (DMTs) in cilia axoneme, which is required for motile cilia beating (PubMed:36191189). Plays a role in spermatogenesis by enhancing the binding of CREM isoform tau to its coactivator FHL5 and increasing the FHL5-regulated transcriptional activation of CREM isoform tau (By similarity). Involved in the acrosome reaction and in binding of sperm to the zona pellucida (By similarity). Plays a role in regulation of the cell cycle by controlling progression through the G2/M phase, possibly by delaying the activation of CDK1 which is required for entry into mitosis (PubMed:19548270). May play a role in fertility and microtubule formation through interaction with RANBP9 (PubMed:10500252). {ECO:0000250|UniProtKB:Q3V0Q6, ECO:0000269|PubMed:10500252, ECO:0000269|PubMed:19548270, ECO:0000269|PubMed:36191189}. This is the function of SPAG8 (sperm associated antigen 8, ENSG00000137098).