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\p=n-\17kDa 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

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... a (BSP) have molecular masses in the range of 15\p=n-\30kDa. 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\p=n-\14kDa) in bovine seminal plama that crossreact with these BSP proteins also show high calmodulin\x=req-\ 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 Ca 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. BSP-A3 and BSP-30 kDa (collectively called BSP proteins; Manjunath and Sairam, 1987; Manjunath et al, 1987) and their tryptic fragments interact with calmodulin. Our previous