Binds sperm in vitro and promotes sperm capacitation (PubMed:22539676, PubMed:24307707). Specifically promotes capacitation induced by high density lipoproteins (HDLs) (PubMed:25602034). Also binds heparin, phospholipid liposomes, and weakly to gelatin (PubMed:22539676). Does not bind chondroitin sulfate B (PubMed:22539676). Expressed only in the epididymis. Belongs to the seminal plasma protein family.