Lutheran (Lu) blood group antigens and the basal cell adhesion molecule antigen reside on two glycoproteins that belong to the Ig superfamily (IgSF) and carry five Ig-like extracellular domains. These glycoproteins act as widely expressed adhesion molecules and represent the unique receptors for laminin-10/11 in erythroid cells. Here, we report the mapping of IgSF domains responsible for binding to laminin. In plasmonic resonance surface experiments, only recombinant Lu proteins containing the N-terminal IgSF domains 1-3 were able to bind laminin-10/11 and to inhibit binding of laminin to Lu-expressing K562 cells. Mutant recombinant proteins containing only IgSF domain 1, domains 1 ؉ 2, domains 1 ؉ 3, domains 2 ؉ 3, domain 3, domain 4, domain 5, and domains 4 ؉ 5 failed to bind laminin as well as a construct containing all of the extracellular domains except domain 3. Altogether, these results indicate that IgSF domains 1-3 are involved in laminin binding and that a specific spatial arrangement of these three first domains is most probably necessary for interaction. Neither the RGD nor the N-glycosylation motifs present in IgSF domain 3 were involved in laminin binding.
Lutheran (Lu)1 blood group antigens and the basal cell adhesion molecule antigen (a marker of epithelial ovarian cancers) are carried by two glycoprotein (gp) isoforms of 85 kDa (Lu) and 78 kDa (Lu(v13)) that are translated from spliceoforms of a single pre-mRNA (1, 2). The Lu gps have a wide tissue distribution with a predominant expression in the basal layer of the epithelium and the endothelium of blood vessel walls (1). Lu and Lu(v13) are type I membrane proteins of the immunoglobulin superfamily (IgSF) that share a similar extracellular domain composed of five Ig-like domains (two V-set and three C2-set) and a single transmembrane domain but differ by the presence or absence of a cytoplasmic C-terminal domain of 40 amino acids (59 versus 19, respectively) (2). The extracellular domain of Lu gps contains a potential integrin binding site in the third IgSF domain as well as five N-glycosylation consensus motifs, one in the third domain and the others in the fourth domain. The 40 C-terminal residues of the Lu gp cytoplasmic tail carry three characteristic motifs involved in protein-protein interaction, polarization to plasma membrane, and intracellular signaling pathways (3), such as (i) a proline-rich motif for potential binding of Src homology 3 domains, (ii) a dileucine motif responsible for the basolateral expression of Lu gp in polarized epithelial cells (4), and (iii) potential phosphorylation motifs. These motifs are absent in the Lu(v13) gp isoform which exhibits a nonpolarized expression in epithelial cells (4). Recent studies have shown that the C-terminal intracellular domain of Lu gps interacts with the membrane skeleton (5, 6).Interestingly, the Lu gps share a significant sequence similarity with the CD146 (MUC18) and CD166 (activated leukocyte cell adhesion molecule) adhesion molecules found in human tissues (7,8), and Lu and Lu(v...