title: Processing of the sperm protein Sp17 during the acrosome reaction and characterization as a calmodulin binding protein.
abstract: In this study we have demonstrated that the native rabbit sperm protein, Sp17, is a 22- to 24-kDa triplet of proteins in washed ejaculated rabbit spermatozoa and is unaffected by capacitation. However, during the acrosome reaction, Sp17 is processed from a 22- to 24-kDa triplet of proteins to a triplet of proteins at 17-19 kDa by the removal of amino acids from the C-terminal. Recombinant rabbit Sp17 (rRSp17) can also be proteolytically processed by acrosome-reacted spermatozoa in a similar manner. Protease inhibitors prevent the proteolytic processing of Sp17. Both forms of native Sp17 remain associated with acrosome-reacted spermatozoa and are solubilized by ionic detergents. Previously, sequence analysis of Sp17 revealed that Sp17 amino acids 108-137 were 52% identical to the calmodulin binding domain of neuromodulin and contained an IQ motif found in other calmodulin binding proteins. In this study, a truncated recombinant Sp17, rRSp17CB, which lacks amino acids 118-146, including the potential calmodulin binding site, was made. Recombinant rabbit Sp17, but not rRSp17CB, binds to calmodulin in the presence of Ca2+ or EDTA, under reduced or nonreduced conditions in biotinylated-calmodulin overlay assays. In DSS crosslinker experiments, calmodulin bound to rRSp17 in a 1:1 ratio but not to rRSp17CB. Additionally, biotinylated rRSp17 interacts with native sperm calmodulin. We propose that the processing of native Sp17, by removing a C-terminal fragment during the acrosome reaction, might be a mechanism to regulate the calmodulin binding activity of Sp17 and provide calmodulin at specific sites after the acrosome reaction.
authors.affiliation: Department of Cell Biology and Anatomy, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina, 27599, USA.
authors.forename: Y
authors.initials: Y
authors.lastname: Wen
authors.forename: R T
authors.initials: RT
authors.lastname: Richardson
authors.forename: M G
authors.initials: MG
authors.lastname: O'rand
mesh_terms: D020101:Acrosome Reaction / Q000502:physiology*; D000595:Amino Acid Sequence; D000818:Animals; D001483:Base Sequence; D001665:Binding Sites / Q000235:genetics; D002147:Calmodulin / Q000378:metabolism; D002148:Calmodulin-Binding Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D002352:Carrier Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D017931:DNA Primers / Q000235:genetics; D066298:In Vitro Techniques; D008297:Male; D008969:Molecular Sequence Data; D008970:Molecular Weight; D010446:Peptide Fragments / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D011485:Protein Binding; D011487:Protein Conformation; D011499:Protein Processing, Post-Translational; D011817:Rabbits; D011994:Recombinant Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D013094:Spermatozoa / Q000378:metabolism*