The α and β Chains of Human Platelet Glycoprotein Ib are Both Transmembrane Proteins Containing a Leucine-Rich Amino Acid Sequence

The primary structure of the β chain of human glycoprotein Ib (GPIb), the platelet receptor for von Willebrand factor, has been established by a combination of cDNA cloning and amino acid sequence analysis. A λ phage cDNA expression library prepared from human erythroleukemia cells (HEL cells) was screened with a radiolabeled affinity-purified rabbit polyclonal antibody to the β chain of GPIb. Eighteen positive clones were isolated and plaque-purified and the nucleotide sequences of three were determined. The composite sequence spanned 968 nucleotides and included a 5′ untranslated region of 22 nucleotides, an open reading frame of 618 nucleotides encoding a signal peptide of 28 amino acids and a mature protein of 181 amino acids, a stop codon, and a 3′ noncoding region of 307 nucleotides. The 3′ noncoding sequence also contained a polyadenylylation signal (AATAAA) 14 nucleotides upstream from the poly(A) tail of 18 nucleotides. Edman degradation of the intact β chain and of peptides produced by chemical cleavage yielded amino acid sequences spanning 76 residues that were identical to those predicted from the cDNA. The amino-terminal region of the β chain contains a leucine-rich sequence of 24 amino acids that is similar to a sequence that occurs as seven tandem repeats in the α chain of GPIb and nine tandem repeats in leucine-rich α 2-glycoprotein. The leucine-rich sequence in the β chain of GPIb is flanked on both sides by amino acid sequences that are similar to those flanking the leucine-rich tandem repeats of the α chain of GPIb and leucine-rich α 2-glycoprotein. The amino-terminal region of the β chain of GPIb is followed by a transmembrane segment of 25 amino acids and an intracellular segment of 34 amino acids at the carboxyl terminus of the protein. The intracellular segment contains an unpaired cysteine and two potential sites for phosphorylation by cAMP-dependent protein kinase.