Analysis of rodent platelet glycoprotein IIb: evidence for evolutionarily conserved domains and alternative proteolytic processing

Recently, the full-length primary amino acid sequence for human glycoproteins (GP) IIb and IIIa have been derived from their respective cDNAs. Potential functional domains within these proteins have been proposed based primarily on homology with similar domains in other proteins having known biologic function. To further understand the relationship between structure and function of the platelet fibrinogen receptor, we have begun comparative studies of the human GPIIb/IIIa receptor with the corresponding rodent receptor. The rodent rGPIIb/IIIa receptor differs from the human receptor, having low affinity for R.G.D- containing oligopeptides and not binding at all to the C-terminus of the gamma chain of human fibrinogen. We describe the structure of rodent platelet GPIIb derived from a combination of peptide sequencing, and cDNA and partial genomic DNA sequence analysis. The initial transcript is 1037 amino acid residues, having 78% amino acid identity with its 1039 residue human analog. Both heavy chains have the N- terminal sequence L.N.L.D, agreeing with the consensus derived from other integrin family alpha heavy chains. All 18 cysteine residues occur at positions conserved in human GPIIb and the vitronectin receptor alpha subunit VNR alpha. The putative calcium-binding domains of the GPIIbs have a high level of amino acid identity (92%), supporting the supposition that these regions have a critical biologic role. The final 48 C-terminal amino acid residues of the heavy chain of rodent GPIIb share only 56% identity with its human counterpart, and the proposed cleavage site of human GPIIb into its heavy and light chains is not present in the rodent sequence. Although we demonstrate that rodent GPIIb is split into two subunits during its maturation, this process either involves a different recognition sequence in the rodent or occurs at a different site. Finally, partial genomic DNA sequence analysis indicates that there are at least two rodent GPIIb genes: a normal gene, containing introns in positions similar to those in the human gene, and a processed pseudogene. The human haploid genome contains only a single GPIIb gene.