title: Structural insight into G-protein coupled receptor binding by apelin.
abstract: Apelin peptides are the cognate ligands for the G-protein coupled receptor APJ, with functions in the cardiovascular and central nervous systems, in glucose metabolism and as a human immunodeficiency virus (HIV-1) coreceptor. Apelin is found in 13-36 residue forms in vivo. The structures of five isoforms of apelin at physiological versus low (5-6 degrees C) temperature are compared here using circular dichroism (CD) and nuclear magnetic resonance (NMR) spectroscopy, demonstrating increased structure at low temperature. Far-ultraviolet (UV) CD spectra are predominantly random coil for apelin isoforms, but are convoluted by unusual bands from the C-terminal phenylalanine side chain. These bands, assigned using F13A-apelin-13, are accentuated at 5 degrees C and imply conformational restriction. At 35 degrees C, the R6-L9 region of apelin-17 is well structured, consistent with previous mutagenesis results showing necessity of this segment for apelin-APJ binding and activation. At 5 degrees C, R6-L9 retains its structuring while the functionally critical C-terminal G13-F17 region also becomes highly structured. Type IV beta-turns and some polyproline-II structure alongside F17 side chain motional reduction correlate well with CD spectral properties. Cis-trans peptide bond isomerization at P14 and P16 produces two sequentially assignable conformers (both trans:both cis approximately 4:1) alongside less populated conformers. Chemical shift assignment of apelin-12, -13 and pyroglutamate-apelin-13 implies highly similar structuring and the same isomerization at the C-terminus. Based on the apelin-17 structure, a two-step binding and activation mechanism is hypothesized.
authors.affiliation: Department of Biochemistry & Molecular Biology, Dalhousie University, Halifax, Nova Scotia B3H 1X5 Canada.
authors.forename: David N
authors.initials: DN
authors.lastname: Langelaan
authors.forename: E Meghan
authors.initials: EM
authors.lastname: Bebbington
authors.forename: Tyler
authors.initials: T
authors.lastname: Reddy
authors.forename: Jan K
authors.initials: JK
authors.lastname: Rainey
mesh_terms: D000595:Amino Acid Sequence; D000596:Amino Acids; D000073861:Apelin; D000075244:Apelin Receptors; D002942:Circular Dichroism; D006801:Humans; D036341:Intercellular Signaling Peptides and Proteins / Q000737:chemistry* / Q000378:metabolism*; D009682:Magnetic Resonance Spectroscopy; D008969:Molecular Sequence Data; D010455:Peptides / Q000737:chemistry / Q000378:metabolism; D011485:Protein Binding; D017433:Protein Structure, Secondary; D043562:Receptors, G-Protein-Coupled / Q000378:metabolism*
chemical_list: C115436:APLN protein, human; C104575:APLNR protein, human; D000596:Amino Acids; D000073861:Apelin; D000075244:Apelin Receptors; D036341:Intercellular Signaling Peptides and Proteins; D010455:Peptides; D043562:Receptors, G-Protein-Coupled