Gene with an Imperfect Polyadenylation Signal Sequence

Glycoprotein (GP) Ib is a major component of the platelet membrane receptor for von Willebrand factor, designated the GP Ib-IX–V complex. GP Ib is composed of two subunits (GP Ib a and GP Ib b ) each synthesized from separate genes. The 206 amino acid precursor of GP Ib b is synthesized from a 1.0-kb mRNA expressed by megakaryocytes and was originally characterized from cDNA clones of human erythroleukemic (HEL) cell mRNA, a cell line exhibiting megakaryocytic-like properties. The cell line CHRF-288–11 also exhibits megakaryocytic-like properties, but synthesizes two related GP Ib b mRNA species of 3.5 and 1.0 kb. We performed cDNA cloning experiments to identify the origin of the 3.5-kb transcript and determine its relationship to the 1.0-kb GP Ib b mRNA found in megakaryocytes, platelets, and HEL cells. Our cloning experiments demonstrate that the longer transcript results from a nonconsensus polyadenylation recognition sequence, 5 9 AACAAT 3 9 , within a separate gene located upstream to the platelet GP Ib b gene. In the absence of normal polyadenylation the more 5 9 gene uses the polyadenylation site within its 3 9 neighbor, the platelet GP Ib b gene. This newly identified 5 9 gene contains an open reading frame encoding 369 amino acids with a high degree of sequence similarity to an expanding family of GTP-binding proteins. ( J. Clin. Invest. 1997. 99:520–525.)