244
J. J. CALVETE and L. SANZ
lemma of acrosome-reacted spermatozoa bound in vivo to the zona pellucida of
oocytes recovered from natural mated sows (Rodriguez-Martinez H, Calvete JJ,
submitted). This represents the first demonstration of a sperm-zona pellucida
binding protein at the site of in vivo fertilisation.
3
HSP-7, a Stallion Seminal Plasma Molecule of the Spermadhesin Family
HSP-7 is a stallion seminal plasma protein, whose primary structure differs in
just three amino acids from that of porcine AWN (Fig. 17.1) (Reinert et al. 1996).
Purified stallion HSP-7, like its porcine homologue, binds isolated equine zonae
pellucidae. This finding, together with the observation that stallion frozen/
thawed epididymal spermatozoa possess fertilising capacity, suggested that spermadhesin HSP-7 may be one of the factors contributing to the reproductive capability of horse epididymal sperm. Moreover, since perissodactyls (e.g. horse) and
artiodactyls (e.g. pig) had a common ancestor over 50 million years ago, the
unusually low mutational rate might suggest that the whole AWN (HSP-7) structure is under strong selective pressure. This points to a highly conserved conformation and, most probably, a common biological function too for AWN and HSP7 in both vertebrate species. Although the actual involvement of HSP-7 in horse
fertilisation deserves further investigation, the homologous zona pellucidabinding activity displayed by the isolated protein supports its classification as a
putative primary sperm-egg adhesion molecule.
The hypothesis that porcine AWN and equine HSP-7 were diverging under
functional constraints, whereas the other polypeptides of the boar seminal
plasma spermadhesin family were diverging more rapidly in their amino acid
sequences, is in line with the proposal that, following gene duplication, one copy
of the gene may divergently have evolved under pressure dictated by the ancestral
function while the duplicate gene(s), unencumbered by a functional role, are free
to search for new functional roles (Trabesinger-Ruef et al. 1996).
4
aSFP and PSP-I/PSP-II Heterodimer: Orphan Spermadhesin Molecules
Acidic seminal fluid protein (aSFP) is the only polypeptide of the spermadhesin
family found in bovine seminal plasma. aSFP displays neither carbohydrate- nor
zona pellucida glycoprotein-binding activity, nor does it bind to the bovine sperm
surface (DostaIova et al. 1994), and may therefore not be involved in sperm-egg
interactions. From the observations that (i) aSFP showed a concentrationdependent protection of bovine sperm from lipid peroxidation in in vitro experiments, and (ii) one of the two disulphide bridges of aSFP displayed re-dox equilibrium, which could account for the protective ability, it has been proposed that the
major function of the bovine spermadhesin would be to act as a buffer dampening
sperm damage caused by active molecules i.e. produced by sperm metabolism and
liberated by occasional sperm death and membrane leakage (Einspanier et al.
1994). Although this attractive hypothesis deserves further study, it indicates that
spermadhesin function has not been conserved during evolution.
J. J. CALVETE and L. SANZ
lemma of acrosome-reacted spermatozoa bound in vivo to the zona pellucida of
oocytes recovered from natural mated sows (Rodriguez-Martinez H, Calvete JJ,
submitted). This represents the first demonstration of a sperm-zona pellucida
binding protein at the site of in vivo fertilisation.
3
HSP-7, a Stallion Seminal Plasma Molecule of the Spermadhesin Family
HSP-7 is a stallion seminal plasma protein, whose primary structure differs in
just three amino acids from that of porcine AWN (Fig. 17.1) (Reinert et al. 1996).
Purified stallion HSP-7, like its porcine homologue, binds isolated equine zonae
pellucidae. This finding, together with the observation that stallion frozen/
thawed epididymal spermatozoa possess fertilising capacity, suggested that spermadhesin HSP-7 may be one of the factors contributing to the reproductive capability of horse epididymal sperm. Moreover, since perissodactyls (e.g. horse) and
artiodactyls (e.g. pig) had a common ancestor over 50 million years ago, the
unusually low mutational rate might suggest that the whole AWN (HSP-7) structure is under strong selective pressure. This points to a highly conserved conformation and, most probably, a common biological function too for AWN and HSP7 in both vertebrate species. Although the actual involvement of HSP-7 in horse
fertilisation deserves further investigation, the homologous zona pellucidabinding activity displayed by the isolated protein supports its classification as a
putative primary sperm-egg adhesion molecule.
The hypothesis that porcine AWN and equine HSP-7 were diverging under
functional constraints, whereas the other polypeptides of the boar seminal
plasma spermadhesin family were diverging more rapidly in their amino acid
sequences, is in line with the proposal that, following gene duplication, one copy
of the gene may divergently have evolved under pressure dictated by the ancestral
function while the duplicate gene(s), unencumbered by a functional role, are free
to search for new functional roles (Trabesinger-Ruef et al. 1996).
4
aSFP and PSP-I/PSP-II Heterodimer: Orphan Spermadhesin Molecules
Acidic seminal fluid protein (aSFP) is the only polypeptide of the spermadhesin
family found in bovine seminal plasma. aSFP displays neither carbohydrate- nor
zona pellucida glycoprotein-binding activity, nor does it bind to the bovine sperm
surface (DostaIova et al. 1994), and may therefore not be involved in sperm-egg
interactions. From the observations that (i) aSFP showed a concentrationdependent protection of bovine sperm from lipid peroxidation in in vitro experiments, and (ii) one of the two disulphide bridges of aSFP displayed re-dox equilibrium, which could account for the protective ability, it has been proposed that the
major function of the bovine spermadhesin would be to act as a buffer dampening
sperm damage caused by active molecules i.e. produced by sperm metabolism and
liberated by occasional sperm death and membrane leakage (Einspanier et al.
1994). Although this attractive hypothesis deserves further study, it indicates that
spermadhesin function has not been conserved during evolution.