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). Interacts with FHL5 (via second LIM domain) (By similarity). Interacts with RANBP9 (PubMed:15014887). In mature sperm cells, detected in the acrosomal region of the head and in the middle piece of the tail (By similarity). Localized to the nucleus and cytoplasm of spermatocytes and round spermatids while, in elongating spermatids, expressed in the cytoplasm but not in the nucleus (By similarity). During the cell cycle, localized on the microtubule-organizing center (MTOC) during prophase. In metaphase, extends along spindle microtubules. In anaphase, detected on the astral microtubules and mid-zone. In telophase, remains at the mid-zone. After cytokinesis, returns to the MTOC (PubMed:19548270). Microtubule inner protein (MIP) part of the dynein-decorated doublet microtubules (DMTs) in cilia axoneme (By similarity). Expressed in testis (germ cells), but not in liver, kidney, prostate and small intestine. May be produced at very low levels due to a premature stop codon in the mRNA, leading to nonsense-mediated mRNA decay. Belongs to the SPAG8 family. Extended C-terminus.