title: The beta-tail domain (betaTD) regulates physiologic ligand binding to integrin CD11b/CD18.
abstract: Crystallographic and electron microscopy studies revealed genuflexed (bent) integrins in both unliganded (inactive) and physiologic ligandbound (active) states, suggesting that local conformational changes are sufficient for activation. Herein we have explored the role of local changes in the contact region between the membrane-proximal beta-tail domain (betaTD) and the ligand-binding betaA domain of the bent conformation in regulating interaction of integrin CD11b/CD18 (alphaMbeta2) with its physiologic ligand iC3b. We replaced the betaTD CD loop residues D658GMD of the CD18 (beta2) subunit with the equivalent D672SSG of the beta3 subunit, with AGAA or with NGTD, expressed the respective heterodimeric receptors either transiently in epithelial HEK293T cells or stably in leukocytes (K562), and measured their ability to bind iC3b and to conformation-sensitive mAbs. In the presence of the physiologic divalent cations Ca(2+) plus Mg(2+) (at 1 mM each), the modified integrins showed increased (in HEK293) or constitutive (in K562) binding to iC3b compared with wild-type receptors. K562 expressing the betaTD-modified integrins bound in Ca(2+)Mg(2+) to the betaA-directed high-affinity reporter mAb 24 but not to mAb KIM127, a reporter of the genu-straightened state. These data identify a role for the membrane proximal betaTD as an allosteric modulator of integrin activation.
authors.affiliation: Nephrology Division, Leukocyte Biology and Inflammation Program, Massachusetts General Hospital, Harvard Medical School, 149 13th Street, Charlestown, MA 02129, USA.
authors.forename: Vineet
authors.initials: V
authors.lastname: Gupta
authors.forename: Annette
authors.initials: A
authors.lastname: Gylling
authors.forename: José Luis
authors.initials: JL
authors.lastname: Alonso
authors.forename: Takashi
authors.initials: T
authors.lastname: Sugimori
authors.forename: Petre
authors.initials: P
authors.lastname: Ianakiev
authors.forename: Jiang-Ping
authors.initials: JP
authors.lastname: Xiong
authors.forename: M Amin
authors.initials: MA
authors.lastname: Arnaout
mesh_terms: D019943:Amino Acid Substitution; D000818:Animals; D039481:CD11b Antigen / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D018821:CD18 Antigens / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D002118:Calcium / Q000737:chemistry / Q000378:metabolism; D003179:Complement C3b / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D006801:Humans; D020014:K562 Cells; D008024:Ligands; D008274:Magnesium / Q000737:chemistry / Q000378:metabolism; D008958:Models, Molecular; D016297:Mutagenesis, Site-Directed; D011485:Protein Binding; D020836:Protein Structure, Quaternary; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary; D013329:Structure-Activity Relationship
chemical_list: D039481:CD11b Antigen; D018821:CD18 Antigens; D008024:Ligands; D003179:Complement C3b; D008274:Magnesium; D002118:Calcium