Important for male fertility. With ROPN1L, involved in fibrous sheath integrity and sperm motility, plays a role in PKA-dependent signaling processes required for spermatozoa capacitation. Homodimer. Interacts with AKAP3 and RHPN1 (By similarity). May interact with SPA17 (PubMed:17551920). Interacts with FSCB; the interaction increases upon spermatozoa capacitation conditions (By similarity). In the sperm tail, found in the principal piece and in the cytoplasmic droplet located at the distal end of the midpiece. Inner surface of the fibrous sheath. Testis specific in adult. Overexpressed in hematologic tumor cells. Expressed in fetal liver. The RIIa domain mediates interaction with AKAP3. Sumoylated, sumoylation decreases upon spermatozoa capacitation conditions. 'Ropporin' comes from the Japanese word 'oppo' which means 'tail'. Belongs to the ropporin family.