A region of the integrin VLAα4 subunit involved in homotypic cell aggregation and in fibronectin but not vascular cell adhesion molecule-1 binding

Abstract

The VLA-4 (α4β1) integrin is involved in the adhesion of cells to fibronectin and vascular cell adhesion molecule-1 (VCAM-1). In order to study α4 structure-function relationships, we have expressed mutated α4 subunit by transfection into VLA-4-negative K562 cells. Substitutions at α4 residues Arg89-Asp90, which show the highest surface probability indexes inside the N-terminal α(4/80) fragment, resulted in a reduction in the reactivity of all anti-α4 epitope A monoclonal antibodies (mAbs) tested, compared with the reactivity with anti-α4 epitopes B1, B2, and C mAb, both by transfectant flow cytometry, and by immunoprecipitation and SDS-polyacrylamide gel electrophoresis analysis of transfectant surface-iodinated proteins. In contrast, substitutions at nearby residues, Gln101, Pro102, and Ile108 did not affect the reactivity of any anti-α4 mAb representing the known α4 epitopes. Homotypic cell aggregation triggered by anti-α4 epitope A mAb was prevented in the transfectants expressing mutated α4 Arg89- Asp90Asp residues, while cell aggregation was fully achieved with either anti-α4 epitope B2 or anti-β1 mAb. Mutations at α4 residues Gln101, Pro102, and Ile108 did not affect the homotypic cell aggregation of the transfectants expressing these mutations. In addition, the adhesion of mutant Arg89-Asp90 α4 transfectants to the connecting segment-1-containing fibronectin-40 (FN-40) fragment of fibronectin was diminished compared to wild type α4 transfectants, as well as to other mutant α4 transfectants. This adhesion to FN-40 was restored when the activating anti-β1 TS2/16 mAb was present in the adhesion assays. In contrast, adhesion to VCAM-1 was not affected by mutations at Arg89-Asp90, nor at Gln101, Pro102, and Ile108 α4 residues. Altogether, these results indicate that α4 residues Arg89 and Asp90 are included in e region involved in homotypic cell aggregation, as well as in adhesion to FN-40, but not to VCAM-1.