Human platelet glycoprotein IX: an adhesive prototype of leucine-rich glycoproteins with flank-center-flank structures

The glycoprotein (GP) Ib-IX complex on the surface of human platelets functions as the von Willebrand factor receptor and mediates von Willebrand factor-dependent platelet adhesion to blood vessels. GPIX is a relatively small (Mr, 17,000) protein that may provide for membrane insertion and orientation of the larger component of the complex, GPIb (Mr, 165,000). Using antibody screening, we cloned a cDNA encoding GPIX from a human erythroleukemia cell cDNA library constructed in phage lambda gt11. Lacking a 5' untranslated region and start codon, the cDNA sequence includes 604 nucleotides, beginning with 495 bases at the 5' end coding for 165 amino acids, followed by a stop codon and 106 noncoding bases at the 3' end. By Northern blot analysis, the GPIX cDNA hybridizes with a single 1.0-kilobase species of platelet poly(A)+ RNA. Translation of the cDNA sequence gives a predicted protein sequence beginning with a truncated putative signal sequence of 5 amino acid followed by a sequence of 17 amino acids matching that determined directly by Edman degradation of intact GPIX. The predicted amino acid sequence of mature GPIX includes an NH2-terminal extracytoplasmic domain of 134 residues, a transmembrane domain of 20 residues, 6 intracytoplasmic residues, and 1 N-linked glycosylation site. GPIX contains a leucine-rich glycoprotein (LRG) sequence of 24 amino acids similar to conserved LRG sequences in GPIb and other proteins from humans, Drosophila, and yeast. "Flanking" sequences of approximately 22 amino acids are present at the NH2 and/or COOH sides of the "central" LRG sequence(s) in GPIX, GPIb, and the other human and Drosophila members of the LRG family. The role of the flank-LRG center-flank structure in the evolution and function of the LRG proteins remains to be defined.