Conformational changes in tertiary structure near the ligand binding site of an integrin I domain.
For efficient ligand binding, integrins must be activated. Specifically, a conformational change has been proposed in a ligand binding domain present within some integrins, the inserted (I) domain [Lee, J., Bankston, L., Arnaout, M. & Liddington, R. C. (1995) Structure (London) 3, 1333-1340]. This proposal remains controversial, however, despite extensive crystal structure studies on the I domain [Lee, J., Bankston, L., Arnaout, M. & Liddington, R. C. (1995) Structure (London) 3, 1333-1340; Liddington, R. & Bankston, L. (1998) Structure (London) 6, 937-938; Qu, A. & Leahy, D. J. (1996) Structure (London) 4, 931-942; and Baldwin, E. T., Sarver, R. W., Bryant, G. L., Jr., Curry, K. A., Fairbanks, M. B., Finzel, B. C. , Garlick, R. L., Heinrikson, R. L., Horton, N. C. & Kelly, L. L. (1998) Structure (London) 6, 923-935]. By defining the residues present in the epitope of a mAb against the human Mac-1 integrin (alphaMbeta2, CD11b/CD18) that binds only the active receptor, we provide biochemical evidence that the I domain itself undergoes a conformational change with activation. This mAb, CBRM1/5, binds the I domain very close to the ligand binding site in a region that is widely exposed regardless of activation as judged by reactivity with other antibodies. The conformation of the epitope differs in two crystal forms of the I domain, previously suggested to represent active and inactive receptor. Our data suggests that conformational differences in the I domain are physiologically relevant and not merely a consequence of different crystal lattice interactions. We also demonstrate that the transition between the two conformational states depends on species-specific residues at the bottom of the I domain, which are proposed to be in an interface with another integrin domain, and that this transition correlates with functional activity.
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Center for Blood Research and Harvard Medical School, Department of Pathology, 200 Longwood Avenue, Boston, MA 02115, USA.
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Springer
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D000595:Amino Acid Sequence; D019943:Amino Acid Substitution; D000818:Animals; D001665:Binding Sites; D002460:Cell Line; D006801:Humans; D020014:K562 Cells; D008024:Ligands; D016177:Macrophage-1 Antigen / Q000737:chemistry* / Q000276:immunology / Q000378:metabolism; D051379:Mice; D008958:Models, Molecular; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D011487:Protein Conformation; D017434:Protein Structure, Tertiary; D011994:Recombinant Proteins / Q000737:chemistry / Q000276:immunology / Q000378:metabolism; D016415:Sequence Alignment; D017386:Sequence Homology, Amino Acid; D014162:Transfection
D008024:Ligands; D016177:Macrophage-1 Antigen; D011994:Recombinant Proteins