Structural Characterisation of Porcine Seminal Plasma Psp-I/Psp-II
245
Porcine seminal plasma (PSP) proteins I and II are major secretory components of the boar seminal vesicle epithelium. Both, PSP-I and PSP-II are glycoproteins and each display site heterogeneity in their single glycosylated residues,
Asn so and Asn 98 , respectively (Solis et al. 1997). In porcine seminal plasma, PSP-II
forms non-covalent heterodimers with specific glycoforms of PSP-I (Calvete et al.
1995b). The PSP-I/PSP-II heterodimer displays carbohydrate- and zona pellucidabinding activity linked to the PSP-II subunit (Calvete et al. 1995b). However, like
bovine aSFP, the PSP-I/PSP-II complex does not bind to the sperm surface excluding its role in gamete interaction. Nonetheless, glycobiological interactions in the
sow's uterus mediated by non-sperm-binding boar seminal plasma lectins might
play a role in other aspects of porcine reproductive physiology. Noteworthy,
although the biological functions of PSP-I/PSP-II remain to be elucidated, Yang et
al. (1998) have provided evidence showing that PSP-I could be involved in the regulation of uterine immune activity. Thus, PSP-I bound to a small (3-5 %) subpopulation of porcine lymphocytes and displayed stimulatory effects on peripheral
lymphocyte activities initiated by pokeweed mitogen (PWM) in a doseresponsive manner, e.g. 600 % increase of [3Hlthymidine uptake at 250 ng/well.
Our own unpublished results (Assreuy AMS, Calvete n, et al.) demonstrate that
intraperitoneal administration of spermadhesin PSP-I/PSP-II acts as a dosedependent inflammatory stimuli inducing neutrophil migration to the site of
inflammation in rats. These findings are parallel to previous observations that
(i) a low molecular mass fraction of porcine seminal plasma enhanced the
growth of lymphocytes induced by PWM, and
(ii) after mating, porcine seminal plasma mediates an inflammatory response in
the female reproductive tract.
Recently, Solis and coworkers (1998) have reported that, in addition to its zona
pellucida glycoprotein- and ~-galactoside-binding site, isolated PSP-II displays a
specific recognition pocket for mannose-6-phosphate and glucose-6-phosphate,
which is distinct but overlaps a binding site for sulphated polysaccharides. Both
sites are cryptic in the heterodimer. Docking of Man-6-P and of a tetrasaccharide
fragment of heparin onto the crystal structure of the PSP-II subunit suggested
that PSP-II Arg 43 , a residue involved in heterodimer formation with PSP-I, may
playa pivotal role in both heparin- and (mannose/glucose)-6-phosphate complexation. The possible biological significance of these cryptic ligand-binding
sites remains to be explored. Hence, the possibility that the cryptic recognition
pockets in the PSP-I/PSP-II heterodimer could be exposed in response to a specific physiological environment cannot be ruled out.
5
The Crystal Structure of Spermadhesin PSP-I/PSP-I! Reveals
the CUB Domain Fold
The crystal structure of porcine seminal plasma spermadhesin PSP-I/PSP-II heterodimer has been determined in two crystal forms by multiple isomorphous
replacement and has been refined at 2.4 A (Varela et al. 1997; Romero et al. 1997).
Although spermadhesin PSP-I/PSP-II heterodimer itself may not participate in
245
Porcine seminal plasma (PSP) proteins I and II are major secretory components of the boar seminal vesicle epithelium. Both, PSP-I and PSP-II are glycoproteins and each display site heterogeneity in their single glycosylated residues,
Asn so and Asn 98 , respectively (Solis et al. 1997). In porcine seminal plasma, PSP-II
forms non-covalent heterodimers with specific glycoforms of PSP-I (Calvete et al.
1995b). The PSP-I/PSP-II heterodimer displays carbohydrate- and zona pellucidabinding activity linked to the PSP-II subunit (Calvete et al. 1995b). However, like
bovine aSFP, the PSP-I/PSP-II complex does not bind to the sperm surface excluding its role in gamete interaction. Nonetheless, glycobiological interactions in the
sow's uterus mediated by non-sperm-binding boar seminal plasma lectins might
play a role in other aspects of porcine reproductive physiology. Noteworthy,
although the biological functions of PSP-I/PSP-II remain to be elucidated, Yang et
al. (1998) have provided evidence showing that PSP-I could be involved in the regulation of uterine immune activity. Thus, PSP-I bound to a small (3-5 %) subpopulation of porcine lymphocytes and displayed stimulatory effects on peripheral
lymphocyte activities initiated by pokeweed mitogen (PWM) in a doseresponsive manner, e.g. 600 % increase of [3Hlthymidine uptake at 250 ng/well.
Our own unpublished results (Assreuy AMS, Calvete n, et al.) demonstrate that
intraperitoneal administration of spermadhesin PSP-I/PSP-II acts as a dosedependent inflammatory stimuli inducing neutrophil migration to the site of
inflammation in rats. These findings are parallel to previous observations that
(i) a low molecular mass fraction of porcine seminal plasma enhanced the
growth of lymphocytes induced by PWM, and
(ii) after mating, porcine seminal plasma mediates an inflammatory response in
the female reproductive tract.
Recently, Solis and coworkers (1998) have reported that, in addition to its zona
pellucida glycoprotein- and ~-galactoside-binding site, isolated PSP-II displays a
specific recognition pocket for mannose-6-phosphate and glucose-6-phosphate,
which is distinct but overlaps a binding site for sulphated polysaccharides. Both
sites are cryptic in the heterodimer. Docking of Man-6-P and of a tetrasaccharide
fragment of heparin onto the crystal structure of the PSP-II subunit suggested
that PSP-II Arg 43 , a residue involved in heterodimer formation with PSP-I, may
playa pivotal role in both heparin- and (mannose/glucose)-6-phosphate complexation. The possible biological significance of these cryptic ligand-binding
sites remains to be explored. Hence, the possibility that the cryptic recognition
pockets in the PSP-I/PSP-II heterodimer could be exposed in response to a specific physiological environment cannot be ruled out.
5
The Crystal Structure of Spermadhesin PSP-I/PSP-I! Reveals
the CUB Domain Fold
The crystal structure of porcine seminal plasma spermadhesin PSP-I/PSP-II heterodimer has been determined in two crystal forms by multiple isomorphous
replacement and has been refined at 2.4 A (Varela et al. 1997; Romero et al. 1997).
Although spermadhesin PSP-I/PSP-II heterodimer itself may not participate in