Critical factors in basal cell adhesion molecule/lutheran-mediated adhesion to laminin

Basal cell adhesion molecule (B-CAM) and Lutheran (LU) are two spliceoforms of a single immunoglobulin superfamily protein containing five Ig domains and comprise the sickle (SS) red cell receptor for laminin. We have now analyzed laminin binding to murine erythroleukemia cells transfected with various human B-CAM/LU constructs. B-CAM and LU bound equally well to laminin, indicating that the longer cytoplasmic tail of LU is not required for binding. However, binding of soluble laminin did require the presence of the membrane-proximal fifth immunoglobulin superfamily (IgSF) domain of LU, while deletion of IgSF domains 1, 2, 3, or 4 individually or together did not abrogate laminin binding. Under flow conditions, MEL cells expressing B-CAM, LU, and LU lacking domains 1, 2, 3, or 4 adhered to immobilized laminin with critical shear stresses over 10 dynes/cm2. However, MEL cells expressing LU lacking domain 5 bound to laminin poorly (critical shear stress = 2.3 dynes/cm2). Moreover, expression of only IgSF domain 5 of LU was sufficient to mediate MEL cell adhesion to immobilized laminin (critical shear stress >10 dynes/cm2). Finally, Scatchard analysis showed that SS red cells had an average of 67% more B-CAM/LU than normal red cells, and low density red cells from sickle cell disease patients expressed 40-55% more B-CAM/LU than high density SS red cells. B-CAM/LU copy number thus may also play a role in the abnormal adhesion of SS red cells to laminin.