The β1 Cytoplasmic Domain Regulates the Laminin-binding Specificity of the α7X1 Integrin

Abstract

During muscle development, the laminin-specific ␣7 integrin is alternatively spliced in the putative ligand-binding domain to yield either the ␣7X1 or the ␣7X2 variant. The relative level of ␣7X1 and ␣7X2 is developmentally regulated. Similarly, the partner ␤1 integrin cytoplasmic domain is converted from the ␤1A to the ␤1D splice variant. To determine whether ␤1D modulates the activity of the ␣7 receptor, cells were transfected with ␣7X1 and ␤1D cDNA. ␣7X1 coupled with ␤1A failed to adhere to laminin-1, whereas cotransfectants expressing ␣7X1 and ␤1D showed strong adhesion. Interestingly, ␣7X1 complexed with ␤1A and ␤1D displayed the same level of poor adhesion to laminin-2/4 or strong adhesion to laminin-10/11. These findings indicate that ␣7 function is regulated not only by X1/X2 in its extracellular domain but also by ␤1 cytoplasmic splice variants. It is likely that expression of ␤1D alters ␣7X1 binding to laminin isoforms by a process related to ligand affinity modulation. Functional regulation of ␣7␤1 by developmentally regulated splicing events may be important during myogenic differentiation and repair because the integrin mediates adhesion, motility, and cell survival.

Distinct Acidic Clusters and Hydrophobic Residues in the Alternative Splice Domains X1 and X2 of α7 Integrins Define Specificity for Laminin Isoforms

“…For example, laminin interactions of α7β1 integrin in muscle are regulated by MIBP, a muscle integrin binding protein, 59 and cell adhesion of α7X1/X2 transfected MCF-7 cells to laminin depends on whether the α7 is associated with β1A or β1D cytoplasmic splice variants. 60 In light of this complex regulation of integrin affinities to their ligands in vivo, elucidation of binding epitopes on the basis of quantitative analysis of changes in the binding affinity of mutants seems only possible in vitro under defined, acelluar conditions with purified reaction partners. Nevertheless, the same mutations (X1E214, X2-D212) that strongly affected laminin binding of the ectodomain heterodimers to laminins in vitro also abolished cell adhesion of K562 cells that were transfected with mutated full-length, transmembrane α7X1/X2 β1 integrins.…”

Genetically Determined Proteolytic Cleavage Modulates α7β1 Integrin Function

“…This suggests that diverse functions of the integrin can be achieved by varying the composition of its extracellular and cytoplasmic domains. The ␤1 cytoplasmic domain may also influence ␣7␤1 binding (23). Additional post-translational modifications, including glycosylation and ADP-ribosylation, may also regulate ␣7␤1 functions (24,25).…”

Modulation of Ligand binding by alternative splicing of the αPS2 integrin subunit

“…Cells expressing a7X1 or a7X2 show similar binding or motility properties on some laminins, but a7X2 is superior on laminin 1 [Ziober et al, 1997;Schöber et al, 2000;von der Mark et al, 2002]. However, a7X1b1 can become an efficient laminin 1 receptor if cells are incubated with activating b1 antibodies or when a7X1 is expressed with the muscle-specific b1D cytoplasmic splice variant [Ziober et al, 1997;Yeh et al, 2003]. These data indicate that like the inclusion of exon 8 in aPS2, the residues of the X1 and X2 exons of a7 do not define cell adhesion specificities only by making specific ligand contacts directly, but also possibly by more global effects on integrin conformation.…”