Structural and functional characterization and physiological significance of a stimulator protein of Mg2+-independent Ca2+-ATPase isolated from goat spermatozoa

Recently a low-molecular-mass protein purified from goat testes cytosol has been reported from our laboratory which is found to stimulate Mg²⁺-independent Ca²⁺-ATPase without any significant effect on Mg²⁺-dependent Ca²⁺-ATPase. In the present study, detailed structural and functional characterization, as well as the physiological significance of the protein has been described. The stimulatory effect is found to be inhibited by known inhibitors of P-type ATPases, vanadate and lanthanum chloride. Monitoring of the phosphoenzyme intermediate by autoradiography has shown that the stimulation of the ATPase is due to the enhancement in the rate of dephosphorylation of the overall reaction step. Along with the stimulation of the enzyme activity, the protein is found to enhance the calcium uptake. Amino acid analysis data show that the stimulator contains about 26% non-polar amino acid facilitating easy penetration to the hydrophobic core of the membrane bound ATPase. Circular dichroism analysis of the protein suggested the presence of all secondary structural elements. The Western-blotting experiment shows its expression level is the highest in goat testes. Peptide fragments obtained in MALDI-MS analysis when subjected to MSDB database search by MASCOT search engine reveals that the proteins of close similarity with the protein under study are actin related protein 2/3 complex subunit, peptidyl-prolyl cis-trans isomerase and gastrin releasing peptide precursor. Besides, the protein under study is also shown to decrease the forward motility of goat sperm without having any significant effect on the total motility indicating its possible role in fertility regulation.