Purification, structural and biophysical characterisation of the major seminal plasma protein from Texel rams

Spermadhesins are a group of low molecular weight proteins present in seminal plasma. In Texel rams, they represent more than 70% of the seminal plasma proteins. Although their functions have not yet been fully clarified, there is much discussion about the role of these proteins in maintaining sperm viability during and after the semen freezing process. This work sought to isolate the major component of the seminal plasma from rams of the Texel breed (O. aries SPD2) and to evaluate its structural and biophysical characteristics in order to better understand its role in spermatic viability. The protein was isolated by centrifugation and ion exchange chromatography and its biophysical properties were evaluated by circular dichroism spectrometry. Molecular dynamics simulations of the modelled protein compared to the homologous bovine protein were also carried out. The results showed that O. aries SPD2 has a transition temperature (T_m) of 65 °C and a ΔH_m of 322.5 kJ mol^-1, similar to the results for other spermadhesins described in the literature. The estimated composition of the secondary structure elements for the native protein is in agreement with that observed for the theoretical model. Its structural characteristics were preserved in simulations at temperatures of 27 °C and 40 °C, as was the case for bull spermadhesin. Taken together, these results suggest that the major component of the spermadhesins of Texel rams (O. aries SPD2) may play an important role in maintaining the viability of spermatozoa in fresh semen as well as after thawing.