title: Three-dimensional structure of guanylyl cyclase activating protein-2, a calcium-sensitive modulator of photoreceptor guanylyl cyclases.
abstract: Guanylyl cyclase activating protein-2 (GCAP-2) is a Ca2+-sensitive regulator of phototransduction in retinal photoreceptor cells. GCAP-2 activates retinal guanylyl cyclases at low Ca2+ concentration (<100 nM) and inhibits them at high Ca2+ (>500 nM). The light-induced lowering of the Ca2+ level from approximately 500 nM in the dark to approximately 50 nM following illumination is known to play a key role in visual recovery and adaptation. We report here the three-dimensional structure of unmyristoylated GCAP-2 with three bound Ca2+ ions as determined by nuclear magnetic resonance spectroscopy of recombinant, isotopically labeled protein. GCAP-2 contains four EF-hand motifs arranged in a compact tandem array like that seen previously in recoverin. The root mean square deviation of the main chain atoms in the EF-hand regions is 2.2 A in comparing the Ca2+-bound structures of GCAP-2 and recoverin. EF-1, as in recoverin, does not bind calcium because it contains a disabling Cys-Pro sequence. GCAP-2 differs from recoverin in that the calcium ion binds to EF-4 in addition to EF-2 and EF-3. A prominent exposed patch of hydrophobic residues formed by EF-1 and EF-2 (Leu24, Trp27, Phe31, Phe45, Phe48, Phe49, Tyr81, Val82, Leu85, and Leu89) may serve as a target-binding site for the transmission of calcium signals to guanylyl cyclase.
authors.affiliation: Center for Advanced Research in Biotechnology, University of Maryland Biotechnology Institute, Rockville, Maryland 20850, USA. james@carb.nist.gov
authors.forename: J B
authors.initials: JB
authors.lastname: Ames
authors.forename: A M
authors.initials: AM
authors.lastname: Dizhoor
authors.forename: M
authors.initials: M
authors.lastname: Ikura
authors.forename: K
authors.initials: K
authors.lastname: Palczewski
authors.forename: L
authors.initials: L
authors.lastname: Stryer
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D002135:Calcium-Binding Proteins / Q000737:chemistry / Q000378:metabolism*; D002417:Cattle; D005136:Eye Proteins; D006162:Guanylate Cyclase / Q000378:metabolism*; D051604:Guanylate Cyclase-Activating Proteins; D051597:Hippocalcin; D008074:Lipoproteins; D009682:Magnetic Resonance Spectroscopy; D008958:Models, Molecular; D008969:Molecular Sequence Data; D009419:Nerve Tissue Proteins / Q000378:metabolism*; D010786:Photoreceptor Cells / Q000201:enzymology*; D011487:Protein Conformation; D011898:Ranidae; D011994:Recombinant Proteins / Q000737:chemistry; D051603:Recoverin
chemical_list: D002135:Calcium-Binding Proteins; D005136:Eye Proteins; D051604:Guanylate Cyclase-Activating Proteins; D008074:Lipoproteins; D009419:Nerve Tissue Proteins; D011994:Recombinant Proteins; D051603:Recoverin; D051597:Hippocalcin; D006162:Guanylate Cyclase; D002118:Calcium