cDNA clones for human platelet GPIIb corresponding to mRNA from megakaryocytes and HEL cells: Evidence for an extensive homology to other Arg‐Gly‐Asp adhesion receptors

Platelet glycoprotein (GP) IIb is one of the two subunits of the common platelet adhesion receptor, GPIIb‐IIIa. The isolation, characterization and sequencing of cDNA clones encoding for the two polypepetide chains of GPIIb are described. A number of clones were isolated from λgt11 libraries constructed with mRNA from an erythroleukemic cell line, HEL, and human megakaryocytes. Two of these clones, λIIb1, from HEL cells, and λIIb2, from megakaryocytes, cross‐hybridized and were selected for detailed analysis. ythe identification of these as authentic GPIIb clones was based on immunological criteria and confirmed by the presence of nucleotide sequences in each insert encoding for known protein sequences of platelet GPIIb. These clones contained inserts of 1.54 kb and 1.39 kb, respectively, with an overlapping sequence of 801 bp. The nucleotide sequence of the overlapping region was identical indicating that HEL cells produce a protein closely related, if not identical, to platelet GPIIb. The determined nucleotide sequence of two inserts included a coding sequence for 648 amino acid residues, a TAG stop codon and 185 nucleotides of 3′ non‐coding sequence followed by a poly(A) tail. The coding sequence contained a portion of the heavy chain, the junction between the heavy and light chains and the entire light chain including a potential transmembrane‐spanning domain and a short cytoplasmic tail. When these cDNA were used to probe for GPIIb mRNA, a single mRNA species of 3.9 kb was identified in both HEL cells and human megakaryocytes. A comparison of the deduced amino acid sequence for GPIIb with those of the α Subunit of the vitronectin and the fibronectin receptors revealed extensive homologies. These homologies further establish that GPIIb‐IIIa from platelets, together with the vitronectin and the fibronectin receptors, are members of a supergene family of adhesion receptors with a recognition specificity for Arg‐Gly‐Asp amino acid sequences.