Calmodulin-binding proteins in bovine semen

Abstract

An 125I-labelled calmodulin gel overlay procedure was used to direct calmodulin-binding proteins in bovine spermatozoa and seminal plasma. Several calmodulin-binding proteins with molecular masses ranging from 12 to > 200 kDa were detected in epididymal and ejaculated spermatozoa. Certain of these proteins exhibited preferential calmodulin-binding in the presence of Ca2+, while others exhibited binding only in its absence. In seminal plasma, only two major proteins with molecular masses of 15 and 16 kDa showed a higher calmodulin-binding activity in the presence of Ca2+, whereas several polypeptides in the range of 6-17 kDa bound higher amounts of radiolabelled calmodulin in the absence of Ca2+. Our previous study has shown that a group of closely related major proteins, designated as BSP-A1, BSP-A2, BSP-A3 and BSP-30 kDa, isolated from bovine seminal plasma (BSP) have molecular masses in the range of 15-30 kDa. This prompted us to investigate whether these polypeptides from bovine seminal fluid interact with calmodulin. The results indicated that calmodulin binds to purified BSP-A1, -A2, -A3 and BSP-30 kDa proteins in the presence and absence of Ca2+. Furthermore, many polypeptides of low molecular mass (6-14 kDa) in bovine seminal plasma that crossreact with these BSP proteins also show high calmodulin-binding activity, particularly in the absence of calcium. This was further demonstrated following the limited proteolysis of the BSP proteins. Several tryptic-peptides of BSP-A1/-A2 and BSP-30 kDa exhibited higher calmodulin-binding activity than the intact BSP proteins. In view of the key role of Ca2+ in triggering the acrosome reaction and the role of calmodulin in intracellular transport of calcium, it is suggested that BSP proteins are involved in sperm capacitation and the acrosome reaction.