Paloma F. Varela

Spermadhesins: A new protein family. Facts, hypotheses and perspectives

Summary Spermadhesins are a novel family of secretory proteins expressed in the male genital tract of pig, horse and bull. They are major products of the seminal plasma and have been found to be peripherally associated to the sperm surface. The structure and function of spermadhesins have been thoroughly investigated in the pig, which exhibits the greatest diversity of members: AWN, AQN‐1, AQN‐2, PSP‐I and PSP‐II and its glycosylated isoforms. They are multifunctional proteins showing a range of ligand‐binding abilities, e.g. carbohydrates, sulfated glycosamino‐glycans, phospholipids and protease inhibitors, suggesting that they may be involved in different steps of fertilization. Isolated porcine spermadhesins bind the zona pellucida glycoproteins in a cation‐dependent manner with a Kd in a low micromolar range, and AWN, AQN‐1 and AQN‐3 display similar binding affinity for glyoproteins containing Galβ(1–3)‐GalNAc and Galβ(1–4)‐GlcNAc sequences in O‐linked and N‐linked oligosaccharides, respectively. During sperm passage through the epididymis AQN‐3 and AWN have been shown to bind tightly to the sperm surface by interaction with the phospholipids of the membrane bilayer. At ejaculation the spermadhesins form a protective coat around the sensitive acrosomal region of the sperm head, thus possibly preventing premature acrosome reaction. During in vitro capacitation most of these aggregated sperm adhesins are lost, with the exception of phospholipid‐bound spermadhesins. AWN and AQN‐3 may now serve as a primary receptor for the oocyte zona pellucida, thus contributing to initial binding and recognition between sperm and egg.

Crystallization and preliminary X-ray diffraction analysis of boar seminal plasma spermadhesin PSP-I/PSP-II, a heterodimer of two CUB domains

Boar spermadhesin PSP‐I/PSP‐II (M r 29 000–30 000), a non‐covalent heterodimer of two CUB domains, was crystallized in two crystal forms. Complete diffraction data sets for hexagonal (space group P61,522) and trigonal (space group P31,221) crystals have been collected up to 2.9 and 2.5 Å resolution, respectively. Cell constants of the hexagonal and trigonal crystal forms are , , and , , respectively. The calculated packing parameters (V m) are 2.8 and 3.2 Å3/Da for the hexagonal and trigonal crystal forms, respectively, indicating that, in both cases, the asymmetric unit is constituted by one PSP‐I/PSP‐II heterodimer. This paper reports the first crystals of a protein built up by a CUB domain architecture.

CRYSTALLIZATION AND PRELIMINARY X-RAY DIFFRACTION ANALYSIS OF BOVINE SEMINAL PLASMA PDC-109, A PROTEIN COMPOSED OF TWO FIBRONECTIN TYPE II DOMAINS

PDC‐109 is a 13 kDa glycoprotein and the major phosphorylcholine‐ and heparin‐binding protein of bull seminal plasma. It is built by an acidic 23‐residue N‐terminal sequence followed by a tandem of fibronectin type II domains. Full‐length PDC‐109 was crystallized in complex with o‐phosphorylcholine by vapor diffusion in sitting drops. Crystals grew to maximal size of 0.5 × 0.3 × 0.2 mm3, diffract x‐rays beyond 2.6 Å resolution, and belong to space group P321 with unit cell dimensions a = b = 93.6 Å, c = 52.7 Å. Proteins 28:454–456, 1997.

X-Ray Crystallographic Analysis of Boar PSP-I/PSP-II Complex

PSP-I and PSP-II are major proteins isolated from porcine seminal plasma (PSP). Both, PSP-I (109 amino acids) and PSP-II (116 amino acids) are glycoproteins and display site heterogeneity, i.e. they contain a single N-glycosylated asparagine residue (PSP-I N50 and PSP-II N98) which accomodates different glycan moieties (Calvete et al.,1993; Calvete et al., 1995а) PSP-II forms a non-covalent heterodimer with certain glycoforms of PSP-I (Calvete et al., 1995а). Both subunits of the heterodimer belong to the spermadhesin protein family (Calvete et al., 1995b). In the pig, this group of proteins include sperm surface-associated lectins that are thought to mediate the initial binding of spermatozoa to carbohydrate structures of zona pellucida glycoproteins (Dostalova et al., 1995; Calvete et al., 1996). The PSP-I/PSP-III heterodimer contains a binding site for zona pellucida glycoproteins located around PSP-II N50 (Calvete et al., 1995а) The zona pellucida-binding characteristics and easiness of isolation of PSP-I/PSP-II (Calvete et al., I 995a), makes this spermadhesin a paradigm for studying protein-carbohydrate interactions involved in gamete recognition. Moreover, spermadhesins are composed of a single CUB domain architecture (Bork et al., 1993) whose three-dimensional structure remains to be determined.