title: Molecular basis of the isoform-specific ligand-binding affinity of inositol 1,4,5-trisphosphate receptors.
abstract: Three isoforms of the inositol 1,4,5-trisphosphate (IP(3)) receptor (IP(3)R), IP(3)R1, IP(3)R2, and IP(3)R3, have different IP(3)-binding affinities and cooperativities. Here we report that the amino-terminal 604 residues of three mouse IP(3)R types exhibited K(d) values of 49.5 +/- 10.5, 14.0 +/- 3.5, and 163.0 +/- 44.4 nm, which are close to the intrinsic IP(3)-binding affinity previously estimated from the analysis of full-length IP(3)Rs. In contrast, residues 224-604 of IP(3)R1 and IP(3)R2 and residues 225-604 of IP(3)R3, which contain the IP(3)-binding core domain but not the suppressor domain, displayed an almost identical IP(3)-binding affinity with a K(d) value of approximately 2 nm. Addition of 100-fold excess of the suppressor domain did not alter the IP(3)-binding affinity of the IP(3)-binding core domain. Artificial chimeric proteins in which the suppressor domain was fused to the IP(3)-binding core domain from different isoforms exhibited IP(3)-binding affinity significantly different from those of the proteins composed of the native combination of the suppressor domain and the IP(3)-binding core domain. Systematic mutagenesis analyses showed that amino acid residues critical for type-3 receptor-specific IP(3)-binding affinity are involved in Glu-39, Ala-41, Asp-46, Met-127, Ala-154, Thr-155, Leu-162, Trp-168, Asn-173, Asn-176, and Val-179. These results indicate that the IP(3)-binding affinity of IP(3)Rs is specifically tuned through the intramolecular attenuation of IP(3)-binding affinity of the IP(3)-binding core domain by the amino-terminal suppressor domain. Moreover, the functional diversity in ligand sensitivity among IP(3)R isoforms originates from at least the structural difference identified on the suppressor domain.
authors.affiliation: Division of Molecular Neurobiology, Department of Basic Medical Sciences, Institute of Medical Science, University of Tokyo, 4-6-1 Shirokanedai, Minato-ku, Tokyo 108-8639, Japan.
authors.forename: Miwako
authors.initials: M
authors.lastname: Iwai
authors.forename: Takayuki
authors.initials: T
authors.lastname: Michikawa
authors.forename: Ivan
authors.initials: I
authors.lastname: Bosanac
authors.forename: Mitsuhiko
authors.initials: M
authors.lastname: Ikura
authors.forename: Katsuhiko
authors.initials: K
authors.lastname: Mikoshiba
mesh_terms: D019943:Amino Acid Substitution; D000818:Animals; D015544:Inositol 1,4,5-Trisphosphate / Q000737:chemistry* / Q000378:metabolism; D053496:Inositol 1,4,5-Trisphosphate Receptors / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D007700:Kinetics; D008024:Ligands; D051379:Mice; D011485:Protein Binding / Q000235:genetics; D020033:Protein Isoforms / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D017434:Protein Structure, Tertiary / Q000235:genetics; D013329:Structure-Activity Relationship
chemical_list: D053496:Inositol 1,4,5-Trisphosphate Receptors; D008024:Ligands; D020033:Protein Isoforms; D015544:Inositol 1,4,5-Trisphosphate