The protein 4.1 family comprises a group of skeletal proteins structurally related to the erythroid membrane skeletal protein, 4.1R, that plays a critical role in determining the morphology and mechanical stability of the red cell plasma membrane. These proteins are characterized by the presence of three main conserved structural/functional domains. A 30-kDa

Lutheran blood group glycoproteins (Lu gps) are receptors for the extracellular matrix protein, laminin. Studies suggest that Lu gps may contribute to vasoocclusion in sickle cell disease and it has recently been shown that sickle cells adhere to laminin isoforms containing the ␣5 chain (laminin 10/11). Laminin ␣5 is present in the subendothelium and is also a constituent of bone marrow sinusoids, suggesting a role for the Lu/ laminin interaction in erythropoiesis. The objectives of the current study were to define more precisely the molecular interactions of the extracellular and intracellular regions of human Lu and to clone and characterize a mouse homologue. To this end, complementary DNA and genomic clones for the mouse homologue were sequenced and the mouse Lu gene mapped to a region on chromosome 7 with conserved synteny with human 19q13.2. Mouse and human Lu gps are highly conserved (72% identity) at the amino acid sequence level and both mouse and human Lu gps specifically bind laminin 10/11 with high affinity. Furthermore, the first 3, N-terminal, immunoglobulin superfamily domains of human Lu are critical for this interaction. The results indicated that the cytoplasmic domain of BRIC 221-labeled human Lu gp is linked with the spectrin-based skeleton, affording the speculation that this interaction may be critical for signal transduction. These results further support a role for Lu gps in sickle cell disease and indicate the utility of mouse models to explore the function of Lu gp-laminin 10/11 interaction in normal erythropoiesis and in sickle cell disease. IntroductionThe Lutheran blood group is composed of a complex set of antigens expressed on 2 integral membrane glycoprotein isoforms of 85 and 78 kd. 1,2 The complementary DNA (cDNA) encoding the 85-kd isoform has been cloned, 3 and the predicted structure is that of a type 1 membrane protein. There are 5 disulfide-bonded, extracellular, immunoglobulin superfamily (IgSF) domains, a single hydrophobic membrane span, and a cytoplasmic tail of 59 residues. 3 The composition of the extracellular IgSF domains puts Lutheran blood group glycoproteins (Lu gps) in the subset of adhesion molecules that includes the human tumor marker MUC18/ MCAM4 and the chicken neural adhesion molecule Gicerin. [4][5][6] Chicken gicerin binds neurite outgrowth factor, a variant of the extracellular matrix (ECM) protein laminin 7,8 and, interestingly, recent studies suggest that Lu gp also functions as a laminin receptor. [9][10][11] The Lu gp cytoplasmic tail contains an SH3 binding motif and 5 potential phosphorylation sites, consistent with receptor signaling function. Of note, differences in the structure of the cytoplasmic tail distinguish the 2 isoforms. The 78-kd isoform (also termed B-CAM 12 or Lu[v13] 13 ) is generated by alternative splicing of intron 13 and differs from the larger form by having a truncated cytoplasmic tail lacking the SH3 binding motif as well as the potential phosphorylation sites. A recent study in epithelial cells 14 suggests that the cytop...