Porcine spermatozoa contain more than one membrane progesterone receptor

Progesterone has been shown to be a physiologically relevant inducer of the sperm acrosome reaction. A novel protein intrinsic to microsomal membranes, membrane progesterone receptor (mPR, now termed progesterone membrane receptor component 1, PGMRC1) that binds progesterone with high affinity has been cloned from porcine liver previously, and corresponding antibodies mitigate the progesterone induced acrosome reaction. In this study we aimed at the localization of mPR in porcine spermatozoa. Immunostaining suggested the exclusive occurrence of mPR in a hardly accessible place, possibly the inner acrosomal membrane, with digitonin dramatically increasing the number of positively stained cells. Consistent with the structure prediction for mPR, its short N-terminus (NT) but not the large C-terminal part becomes accessible from outside after digitonin treatment as evidenced by the staining pattern of antibodies directed against different regions of the protein. However, digitonin treatment solubilizes a progesterone binding activity of approximately 140 kDa molecular weight, that is different from mPR, which remains in the cell membrane as demonstrated by Western blotting. Ligand binding studies confirm the dissimilarity of mPR and the digitonin-soluble progesterone binding protein. Chemical modification studies also indicate that the digitonin-soluble progesterone binding protein has a binding site that differs from that of mPR. It is concluded that more than one progesterone receptor is present in porcine spermatozoa.