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 α chains have been implicated in various cellular interactions that are mediated through receptors such as integrins, α-dystroglycan, syndecans, and the Lutheran blood group glycoprotein (Lu). Lu, an Ig superfamily transmembrane receptor specific for laminin α5, is also known as basal cell adhesion molecule (B-CAM). Although Lu/B-CAM binds to the LG domain of laminin α5, the binding site has not been precisely defined. To better delineate this binding site, we produced a series of recombinant laminin trimers containing modified α chains, such that all or part of α5LG was replaced with analogous segments of human laminin α1LG. 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 α5 domains LN through LG3 fused to human laminin α1LG4-5. However, Lu/B-CAM did not bind other recombinant laminins containing α5LG3 unless α5LG1-2 was also present. A recombinant α5LG1-3 tandem lacking the laminin coiled coil (LCC) domain did not reproduce the activity of Lu/B-CAM binding. Therefore, proper structure of the α5LG1-3 tandem with the LCC domain was essential for the binding of Lu/BCAM to laminin α5. Our results also suggest that the binding site for Lu/B-CAM on laminin α5 may overlap with that of integrins α3β1 and α6β1.