beta1 Integrin and alpha-dystroglycan binding sites are localized to different laminin-G-domain-like (LG) modules within the laminin alpha5 chain G domain

Laminins are a group of extracellular-matrix proteins important in development and disease. They are heterotrimers, and specific domains in the different chains have specialized functions. The G domain of the α5 chain has now been produced in transfected mammalian cells as single modules and two tandem arrays, α5LG1–3 and α5LG4–5 (LG is laminin G domain-like). Using these fragments we produced specific polyclonal antibodies functional in immunoblotting and immunofluorescence studies and in solid-phase assays. Both α5LG tandem arrays had physiologically relevant affinities for sulphated ligands such as heparin and sulphatides. Cells adhered to these fragments and acquired a spread morphology when plated on α5LG1–3. Binding of integrins α3β1 and α6β1 was localized to the α5LG1–3 modules, and α-dystroglycan binding was localized to the α5LG4–5 modules, thus locating these activities to different LG modules within the laminin α5 G domain. However, both these activities were of relatively low affinity, indicating that integrin-mediated cell adhesion to the laminin 10/11 α5G domain depends on contributions from the other chains of the heterotrimer and that high-affinity α-dystroglycan binding could be dependent on specific Ca²⁺-ion-co-ordinating amino acids absent from α5LG4–5.