title: Progesterone modulation of transmembrane helix-helix interactions between the alpha-subunit of Na/K-ATPase and phospholipid N-methyltransferase in the oocyte plasma membrane.
abstract: BACKGROUND
  Progesterone binding to the surface of the amphibian oocyte initiates the meiotic divisions. Our previous studies with Rana pipiens oocytes indicate that progesterone binds to a plasma membrane site within the external loop between the M1 and M2 helices of the alpha-subunit of Na/K-ATPase, triggering a cascade of lipid second messengers and the release of the block at meiotic prophase. We have characterized this site, using a low affinity ouabain binding isoform of the alpha1-subunit.
  
  
  RESULTS
  Preparations of isolated plasma membranes from Rana oocytes demonstrate that physiological levels of progesterone (or the non-metabolizable progestin R5020) successively activate phosphatidylethanolamine-N-methyltransferase (PE-NMT) and sphingomyelin synthase within seconds. Inhibition of PE-NMT blocks the progesterone induction of meiosis in intact oocytes, whereas its initial product, phosphatidylmonomethylethanolamine (PME), can itself initiate meiosis in the presence of the inhibitor. Published X-ray crystallographic data on Na/K-ATPase, computer-generated 3D projections, heptad repeat analysis and hydrophobic cluster analysis of the transmembrane helices predict that hydrophobic residues L, V, V, I, F and Y of helix M2 of the alpha1-subunit interact with F, L, G, L, L and F, respectively, of helix M3 of PE-NMT.
  
  
  CONCLUSIONS
  We propose that progesterone binding to the first external loop of the alpha1-subunit facilitates specific helix-helix interactions between integral membrane proteins to up-regulate PE-NMT, and, that successive interactions between two or more integral plasma membrane proteins induce the signaling cascades which result in completion of the meiotic divisions.
authors.affiliation: Department of Physiology and Biophysics, Albert Einstein College of Medicine, Bronx, New York 10461, USA. DrgeneM@aol.com
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: D000818:Animals; D002462:Cell Membrane / Q000201:enzymology*; D018360:Crystallography, X-Ray; D057927:Hydrophobic and Hydrophilic Interactions; D008540:Meiosis; D009865:Oocytes / Q000201:enzymology*; D050941:Phosphatidyl-N-Methylethanolamine N-Methyltransferase / Q000737:chemistry* / Q000378:metabolism; D011374:Progesterone / Q000737:chemistry* / Q000494:pharmacology; D011485:Protein Binding; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary; D021122:Protein Subunits; D011898:Ranidae; D000254:Sodium-Potassium-Exchanging ATPase / Q000737:chemistry* / Q000378:metabolism; D017855:Transferases (Other Substituted Phosphate Groups) / Q000378:metabolism
chemical_list: D021122:Protein Subunits; D011374:Progesterone; D050941:Phosphatidyl-N-Methylethanolamine N-Methyltransferase; D017855:Transferases (Other Substituted Phosphate Groups); C070186:phosphatidylcholine-ceramide phosphocholine transferase; D000254:Sodium-Potassium-Exchanging ATPase