The LG1-3 tandem of laminin α5 harbors the binding sites of lutheran/basal cell adhesion molecule and α3β1/α6β1 integrins

The laminin-type globular (LG) domains of laminin alpha chains have been implicated in various cellular interactions that are mediated through receptors such as integrins, alpha-dystroglycan, syndecans, and the Lutheran blood group glycoprotein (Lu). Lu, an Ig superfamily transmembrane receptor specific for laminin alpha 5, is also known as basal cell adhesion molecule (B-CAM). Although Lu/B-CAM binds to the LG domain of laminin alpha 5, the binding site has not been precisely defined. To better delineate this binding site, we produced a series of recombinant laminin trimers containing modified alpha chains, such that all or part of alpha 5LG was replaced with analogous segments of human laminin alpha 1LG. In solid phase binding assays using a soluble Lu (Lu-Fc) composed of the Lu extracellular domain and human IgG1 Fc, we found that Lu bound to Mr5G3, a recombinant laminin containing alpha 5 domains LN through LG3 fused to human laminin alpha 1LG4-5. However, Lu/B-CAM did not bind other recombinant laminins containing alpha 5LG3 unless alpha 5LG1-2 was also present. A recombinant alpha 5LG1-3 tandem lacking the laminin coiled coil (LCC) domain did not reproduce the activity of Lu/B-CAM binding. Therefore, proper structure of the alpha 5LG1-3 tandem with the LCC domain was essential for the binding of Lu/BCAM to laminin alpha 5. Our results also suggest that the binding site for Lu/B-CAM on laminin alpha 5 may overlap with that of integrins alpha 3 beta 1 and alpha 6 beta 1.