Structure of Platelet Glycoprotein

The platelet membrane glycoprotein lIb/IlIa complex is a member of a family of a/,8 heterodimers that function as receptors for adhesive proteins. In this report we describe the structure of the human ft subunit GPIIIa deduced from an analysis of 4.0 kb of overlapping cDNA sequences isolated from a human erythroleukemia (HEL) cell cDNA expression library. A continuous open reading frame encoding all 788 amino acids for GPIIIa was present. The deduced amino acid sequence included a 26-residue amino-terminal signal peptide, a 29-residue transmembrane domain near the carboxy terminus, and four tandemly repeated cysteine-rich domains of 33-38 residues. An exact correspondence of 128 amino acids from seven human platelet GPIIIa fragments with HEL GPIIIa indicates that HEL and platelet GPIIIa are the same gene product. The HEL GPIIIa sequence was compared with the sequences of the ft subunit for the human LFA-1/Mac-l/ p150,95 complex and human endothelial cell GPIIIa, revealing a 38% similarity with the former and virtual identity with the latter. Northern blot analysis using RNA from both HEL and endothelial cells revealed two GPIIIa transcripts of 5.9 and 4.1 kb. However, HEL RNA, but not endothelial cell RNA, contained a transcript for GPIIb. This indicates that the GPIIIacontaining heterodimers in platelets and endothelial cells are not identical structures, but are members of a subfamily within the human family of adhesion protein receptors sharing an identical ft subunit. Introduction The platelet membrane glycoprotein Ilb/IIIa (GPIIb/IIIa)' complex is a calcium-dependent heterodimer containing binding sites for fibrinogen, vWf, fibronectin, and vitronectin that are exposed by platelet activation (1-4). GPIIb/IIIa is also a member of a family ofadhesive protein receptors that includes chick fibroblast integrin, the LFA-l/Mac-l/pl50,95 comAddress reprint requests to Dr. Mortimer Poncz, Division ofHematology, The Children's Hospital of Philadelphia, 34th Street and Civic Center Blvd., Philadelphia, PA 19104. Received for publication 27 August 1987 and in revised form 13 November 1987. 1. Abbreviations used in this paper: GP, glycoprotein; HEL, human erythroleukemia. plexes of leukocytes, the VLA complexes present on stimulated T lymphocytes, and the fibronectin and vitronectin receptors found on a variety of cells (5). Biochemical examination indicates that they are afl3 heterodimers with a subunits analogous to GPIIb, and ,3 subunits analogous to GPIIIa (5) and many interact with ligands containing the Arg-Gly-Asp sequence (4, 6). Recently, the amino acid sequences for several a and # subunits have been determined by analysis of cDNA and have demonstrated homology among the various a and among the various fi subunits (7-12). We report here the structure of GPIIIa deduced from an analysis of cDNA from a human erythroleukemia (HEL) cell expression library. The HEL cell line originated in a patient with erythroleukemia and has megakaryocyte-like properties, constitutively expressing a number of platelet proteins such as platelet factor 4, f3-thromboglobulin, vWf, GPIb, and GPIIb/ IlIa (13, 14). Recently, we reported the isolation ofcDNA for platelet factor 4, a megakaryocyte-specific protein, and for GPIIb from this HEL library (7, 15). Comparison of the deduced sequence ofHEL GPIIb with known partial amino acid sequences of platelet GPIIb demonstrated that HEL GPIIb is identical to platelet GPIIb (16, 17). The results described in the present paper indicate that HEL GPIIIa and platelet GPIIIa also appear to be products of the same gene and are essentially identical to the GPIIIa expressed by endothelial cells. In addition, we demonstrate that endothelial cells do not express a gene for GPIIb. These findings indicate that the adhesive protein receptors expressed by platelets and endothelial cells are distinct structures, but share a common (3 subunit.