High levels of a calcium-dependent modulator protein in spermatozoa and its similarity to brain modulator protein

Sperm from hamster, human, rooster, rabbit and sea urchin were found to contain relatively high levels of calcium-dependent modulator protein. Using rabbit sperm the modulator protein was found to be exclusively located in the sperm head fraction (nuclei + acrosomes) with no activity present in the midpiece or tail regions. The modulator protein represents approximately 12% of the total soluble protein found in the sperm head fraction and is similar to porcine and brain modulator proteins in its ability to activate brain cyclic nucleotide phosphodiesterase, its heat stability and electrophoretic migration. We have also observed modulator protein to be present in high levels in sea urchin eggs.