title: Progesterone binding to the alpha1-subunit of the Na/K-ATPase on the cell surface: insights from computational modeling.
abstract: Progesterone triggers the resumption of meiosis in the amphibian oocyte through a signaling system at the plasma membrane. Analysis of [(3)H]ouabain and [(3)H]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 112kDa alpha1-subunit of the membrane Na/K-ATPase. Published amino acid sequences from both low and high affinity ouabain binding alpha1-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 beta and hydrophobic alpha 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 Arg(118), Asp(129) and possibly Glu(122-124) in the C-terminal region of the loop. The 20-keto group hydrogen may in turn hydrogen bond to Cys(111) 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.
authors.affiliation: Department of Physiology & Biophysics, Albert Einstein College of Medicine, Bronx, NY 10461, USA. morrill@aecom.yu.edu
authors.forename: Gene A
authors.initials: GA
authors.lastname: Morrill
authors.forename: Adele B
authors.initials: AB
authors.lastname: Kostellow
authors.forename: Amir
authors.initials: A
authors.lastname: Askari
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D001665:Binding Sites; D002462:Cell Membrane / Q000378:metabolism*; D003198:Computer Simulation; D018360:Crystallography, X-Ray; D005260:Female; D009682:Magnetic Resonance Spectroscopy; D008954:Models, Biological; D008958:Models, Molecular; D008969:Molecular Sequence Data; D015394:Molecular Structure; D009865:Oocytes / Q000378:metabolism; D010042:Ouabain / Q000737:chemistry / Q000378:metabolism; D011374:Progesterone / Q000737:chemistry / Q000378:metabolism*; D011485:Protein Binding; D011894:Rana pipiens; D017386:Sequence Homology, Amino Acid; D000254:Sodium-Potassium-Exchanging ATPase / Q000737:chemistry / Q000378:metabolism*
chemical_list: D011374:Progesterone; D010042:Ouabain; D000254:Sodium-Potassium-Exchanging ATPase