Alternative expression of platelet glycoprotein Ibβ mRNA from an adjacent 5' gene with an imperfect polyadenylation signal sequence

Abstract

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 Ibα and GP Ibβ) each synthesized from separate genes. The 206 amino acid precursor of GP Ibβ 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 Ibβ 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 Ibβ mRNA found in megakaryocytes, platelets, and HEL cells. Our cloning experiments demonstrate that the longer transcript results from a nonconsensus polyadenylation recognition sequence, 5'AACAAT3', within a separate gene located upstream to the platelet GP Ibβ gene. In the absence of normal polyadenylation the more 5' gene uses the polyadenylation site within its 3' neighbor, the platelet GP Ibβ gene. This newly identified 5' 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.