Identification of essential GATA and Ets binding motifs within the promoter of the platelet glycoprotein Ib alpha gene.

Platelet glycoprotein (GP) Ib-IX-V is a multisubunit adhesion receptor that supports platelet attachment to thrombogenic surfaces at sites of vascular injury. The congenital absence of the receptor results in a bleeding disorder associated with "giant" platelets, a condition linking the expression of the complex to platelet morphogenesis. To understand better the expression of the GP Ib-IX-V complex, studies were undertaken to define the essential genetic elements supporting the expression of the alpha-subunit of the complex (GP Ib alpha). GP Ib alpha promoter activity was evaluated by transfection of human erythroleukemia cells with reporter plasmids coding for the enzyme, luciferase. Studies were initiated with a fragment extending 2,738 nucleotides 5' to the transcription start site and lead to the identification of 253 nucleotides retaining full promoter activity in human erythroleukemia cells. In cells of nonhematopoietic lineage, human endothelial and HeLa cells, the GP Ib alpha promoter activity was no greater than background levels obtained with promoterless constructs. Gel shift assays and site-directed mutagenesis studies defined essential GATA and Ets binding motifs 93 and 150 nucleotides upstream of the transcription start site, a finding which further substantiates these elements as important determinants of megakaryocytic gene expression. The results define essential cis-acting elements responsible for the expression of GP Ib alpha and provide insights into molecular events coinciding with the release of normal platelets into the bloodstream.