Progesterone binding to the α1-subunit of the Na/K-ATPase on the cell surface: Insights from computational modeling

Progesterone triggers the resumption of meiosis in the amphibian oocyte through a signaling system at the plasma membrane. Analysis of [3H]ouabain and [3H]progesterone binding to the plasma membrane of the Rana pipiens oocyte indicates that progesterone competes with ouabain for a low affinity ouabain binding site on a 112 kDa α1-subunit of the membrane Na/K-ATPase. Published amino acid sequences from both low and high affinity ouabain binding α1-subunits are compared, together with published site-directed mutagenesis studies of ouabain binding. We propose that the progesterone binding site is located in the external loop (23 amino acids) between the M1-M2 transmembrane helices. Analysis of loop topology and the countercurrent hydrophobicity/polarity gradients within the M1-M2 loop further suggest that the polar β and hydrophobic α surfaces of the planar progesterone molecule interact with opposite sides of the amino acid loop. The 19-angular methyl group of progesterone is essential for activity; it could bind to the C-terminal region of the M1-M2 loop. Maximum biological activity requires formation of hydrogen-bond networks between the 3-keto group of progesterone and Arg118, Asp129 and possibly Glu122–124 in the C-terminal region of the loop. The 20-keto group hydrogen may in turn hydrogen bond to Cys111 near the M1 helix. Peptide flexibility undergoes a maximal transition near the midway point in the M1-M2 loop, suggesting that folding occurs within the loop, which further stabilizes progesterone binding.