Seminalplasmin. An endogenous calmodulin antagonist

Interaction of antimicrobial peptides with biological and model membranes: structural and charge requirements for activity

Species right across the evolutionary scale from insects to mammals use peptides as part of their host-defense system to counter microbial infection. The primary structures of a large number of these host-defense peptides have been determined. While there is no primary structure homology, the peptides are characterized by a preponderance of cationic and hydrophobic amino acids. The secondary structures of many of the host-defense peptides have been determined by a variety of techniques. The acyclic peptides tend to adopt helical conformation, especially in media of low dielectric constant, whereas peptides with more than one disulfide bridge adopt beta-structures. Detailed investigations have indicated that a majority of these host-defense peptides exert their action by permeabilizing microbial membranes. In this review, we discuss structural and charge requirements for the interaction of endogenous antimicrobial peptides and short peptides that have been derived from them, with membranes.

Seminal plasmin

The importance of seminal plasma in fertilization was appreciated as early as 1677 and would thus hardly seem a source for the search of antibacterial agents. The observation that seminal plasma had the ability to inhibit the growth of microorganisms in 1940 led to a systematic search for molecules possessing antimicrobial activity in addition to factors that might have a role in reproductive physiology. Extensive investigations led to the discovery in bovine seminal fluid of a 47-residue peptide, possessing potent antimicrobial activity as well as calcium transport modulatory properties in bovine sperm. We describe in this article the two, apparently unrelated, biological activities of this peptide.

A simple and efficient method for the purification of seminal plasmin, an antimicrobial protein from bovine seminal plasma

A method involving direct affinity chromatography of undialysed bovine seminal plasma on a calmodulin-agarose column was developed for the purification of seminalplasmin. Seminalplasmin thus obtained was further purified from any contaminants left by ion-exchange chromatography on a short CM-Sepharose column. The method gave an excellent yield of seminalplasmin (0.7 mg per ml seminal plasma) that showed 100% inhibition of bacterial growth at a concentration of 10 micrograms/ml. The purified seminalplasmin was found to be homogeneous as tested by HPLC on a reverse-phase column and SDS-PAGE.

Seminalplasmin: a protein with many biological properties

Proteins present in the seminal plasma of mammals are known to influence functions associated with ejaculated spermatozoa such as motility, capacitation, acrosome reaction and fertilising ability. The proteins isolated and characterised so far influence only one of the above functions of spermatozoa. Seminalplasmin, a protein isolated from the seminal plasma of bull is exceptional in that it influences many of the above spermatozoal functions. It is also a potent antimicrobial protein and capable of lysing microbial and mammalian cells. The physiological function of seminalplasmin as nature's own antifertility agent is discussed.

The major protein of bull seminal plasma: biosynthesis and biological function

We isolated the major protein of apparent Mr of 15,000-16,000 from seminal plasma as well as from seminal vesicle secretion of bull and proved by amino acid analysis and tryptic peptide mapping that the two proteins were identical. An antiserum against this major protein was employed to quantitate and identify the major protein in seminal plasma as well as seminal vesicle secretion. The antiserum did not cross-react with proteins from bovine or human plasma or follicular fluid respectively. Cell-free translation of poly(A)RNA from seminal vesicle tissue and immunoprecipitation yielded one major species with apparent Mr of 18,000. Using the anti-major protein antiserum, this major species was specifically immuno absorbed. Cloning and sequencing of a major protein-specific cDNA led to the identification of clone pMP17, encoding a precursor of the major protein of 128 amino acid residues. We proved that the major protein is identical to protein PDC 109 (Esch et al., Biochem. Biophys. Res. Comm. 113:861-867, 1983). The seminal vesicles synthesize major protein in an androgen-dependent fashion. In addition to intraluminal secretion of the vas deferens, ampullary spermatozoa revealed an intense immunoreaction which was restricted to the neck region of the sperm head and the middle piece, while the principal piece of the tail as well as the sperm head were devoid of immunoreactive material. Epididymal epithelium (as well as calf seminal vesicle epithelium) showed no immunoreactivity with major protein antiserum. Immunoelectron microscopy demonstrated that only spermatozoa devoid of a plasma membrane around the middle piece were able to bind the antiserum against major protein. After removal of the plasma membrane from epididymal spermatozoa, binding of major protein to subplasmalemmal binding sites was visualised using gold-labeled MP. Transblotting with gold-labeled MP demonstrated a protein of about 66 kDa which appears to represent the major protein-receptor. Binding of major protein to the receptor (after loss of the plasma membrane in the mid-piece region of the spermatozoa after contact with secretions from seminal vesicles) is interpreted as a physiological process presumably related to the onset of sperm motility.

Affinity purification of seminalplasmin and characterization of its interaction with calmodulin

Bull seminalplasmin antagonizes with high potency and selectivity the activating effect of calmodulin on target enzymes [Gietzen & Galla (1985) Biochem. J. 230, 277-280]. In the present paper we establish that seminalplasmin forms a 1:1, Ca2+-dependent and urea-resistant complex with calmodulin. The dissociation constant equals 1.6 nM. In the absence of Ca2+ a low-affinity complex is formed that is disrupted by 4 M-urea. On the basis of these properties, a fast affinity purification of seminalplasmin was developed. The high specificity of seminalplasmin as a calmodulin antagonist was demonstrated for the multipathway-regulated adenylate cyclase of bovine cerebellum. Far-u.v. c.d. properties are consistent with a random form of seminalplasmin in aqueous solution; 23% alpha-helix is induced on interaction with calmodulin. The fluorescence properties of the single tryptophan residue of seminalplasmin are markedly changed on formation of the complex. These studies allowed us to locate tentatively the peptide segment that interacts with calmodulin, and to ascertain the structural homology between seminalplasmin and other calmodulin-binding peptides. Additional material, showing the inhibition of calmodulin-mediated activation of bovine brain phosphodiesterase by melittin and seminalplasmin and also the near-u.v. spectrum of affinity-purified seminalplasmin, has been deposited as supplement SUP 50135 (4 pages) at the British Library Lending Division, Boston Spa, Wetherby, West Yorkshire LS23 7BQ, U.K., from whom copies may be obtained on the terms indicated in Biochem. J. (1986) 233, 5.

Interaction of GTP-binding proteins with calmodulin

Two GTP-binding proteins (Gi and Go), which were the substrates for islet-activating protein, pertussis toxin, were purified from bovine cerebral cortical membranes. Both Gi and Go completely inhibited calmodulin-stimulated cyclic nucleotide phosphodiesterase activity. The same concentrations of these proteins, however, had no appreciable effect on the basal phosphodiesterase activity. The isolated Gi alpha and beta gamma subunits of GTP-binding proteins were potent inhibitors of the calmodulin-stimulated phosphodiesterase activity, but Go alpha was very weak. Therefore, the beta gamma subunits were likely to be the major active molecules in the brain membranes. GTP-binding proteins were shown to bind directly to calmodulin in a Ca2+-dependent manner by a gel permeation binding experiment.

Seminalplasmin and caltrin are the same protein

The sequence of seminalplasmin, a basic antimicrobial and transcription-inhibitory protein from bovine seminal plasma, has been determined using an automated sequenator. This sequence is slightly different from that reported earlier by Theil and Scheit [(1983) EMBO J. 2, 1159-1163] and identical with that of caltrin, a Ca2+-transport-inhibitory protein of bovine seminal plasma. Caltrin and seminalplasmin are, therefore, the same protein.