title: Crystal structure of the annexin XII hexamer and implications for bilayer insertion.
abstract: Annexins are a family of calcium- and phospholipid-binding proteins implicated in a number of biological processes including membrane fusion and ion channel formation. The crystal structure of the annexin XII hexamer, refined at 2.8 A resolution, forms a concave disk with 3-2 symmetry, about 100 A in diameter and 70 A thick with a central hydrophilic pore. Six intermolecular Ca2+ ions are involved in hexamer formation. An additional 18 Ca2+ ions are located on the perimeter of the disk, accessible only from the side of the hexameric disk. On the basis of the hexamer structure we propose here a new mode of protein-phospholipid bilayer interaction that is distinct from the hydrophobic insertion of typical membrane proteins. This speculative model postulates the Ca(2+)-dependent insertion of the hydrophilic annexin XII hexamer into phospholipid bilayers with local reorientation of the bilayer phospholipids.
mesh_terms: D000818:Animals; D017302:Annexins / Q000737:chemistry* / Q000378:metabolism; D002118:Calcium / Q000378:metabolism; D018360:Crystallography, X-Ray; D006829:Hydra; D008051:Lipid Bilayers; D008563:Membrane Lipids / Q000737:chemistry / Q000378:metabolism; D008565:Membrane Proteins / Q000737:chemistry* / Q000378:metabolism; D008958:Models, Molecular; D010743:Phospholipids / Q000737:chemistry / Q000378:metabolism; D011487:Protein Conformation; D011994:Recombinant Proteins / Q000737:chemistry
chemical_list: D017302:Annexins; D008051:Lipid Bilayers; D008563:Membrane Lipids; D008565:Membrane Proteins; D010743:Phospholipids; D011994:Recombinant Proteins; D002118:Calcium