Multiple loop structures critical for ligand binding of the integrin alpha4 subunit in the upper face of the beta-propeller mode 1.
A non-I-domain integrin, alpha4beta1, recognizes vascular cell adhesion molecule 1 (VCAM-1) and the IIICS portion of fibronectin. To localize regions of alpha4 critical for ligand binding, we swapped several predicted loops within or near the putative ligand-binding site of alpha4 (which spans repeats 2-5 of the seven N-terminal repeats) with the corresponding regions of alpha5. Swapping residues 112-131 in repeat 2, or residues 237-247 in repeat 4, completely blocked adhesion to immobilized VCAM-1 and connecting segment 1 (CS-1) peptide. However, swapping residues 40-52 in repeat 1, residues 151-164 in repeat 3, or residues 282-288 (which contain a putative cation binding motif) in repeat 5 did not affect or only slightly reduced adhesion to these ligands. The binding of several function-blocking antibodies is blocked by swapping residues 112-131, 151-164, and 186-191 (which contain previously identified residues critical for ligand binding, Tyr-187 and Gly-190). These results are consistent with the recently published beta-propeller folding model of the integrin alpha4 subunit [Springer, T. A. (1997) Proc. Natl. Acad. Sci. USA 94, 65-72], in which seven four-stranded beta-sheets are arranged in a torus around a pseudosymmetric axis. The regions of alpha4 critical for ligand binding are adjacent to each other and are located in the upper face, the predicted ligand-binding site, of the beta-propeller model, although they are not adjacent in the primary structure.
Irie
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A
Department of Vascular Biology, The Scripps Research Institute, 10550 North Torrey Pines Road, La Jolla, CA 92037, USA.
Kamata
T
T
Takada
Y
Y
D000595:Amino Acid Sequence; D001665:Binding Sites; D005353:Fibronectins / Q000737:chemistry; D006801:Humans; D039041:Integrin alpha4beta1; D016023:Integrins / Q000737:chemistry* / Q000235:genetics; D008024:Ligands; D008969:Molecular Sequence Data; D009154:Mutation; D016201:Receptors, Lymphocyte Homing / Q000737:chemistry* / Q000235:genetics; D017421:Sequence Analysis; D019010:Vascular Cell Adhesion Molecule-1 / Q000737:chemistry*
D005353:Fibronectins; D039041:Integrin alpha4beta1; D016023:Integrins; D008024:Ligands; D016201:Receptors, Lymphocyte Homing; D019010:Vascular Cell Adhesion Molecule-1