AN ETS SP1 INTERACTION IN THE 5'-FLANKING REGION OF THE MEGAKARYOCYTE-SPECIFIC ALPHA-IIB GENE APPEARS TO STABILIZE SP1 BINDING AND IS ESSENTIAL FOR EXPRESSION OF THIS TATA-LESS GENE/

The megakaryocyte-specific integrin, alpha IIb, is the alpha-subunit o f the alpha IIb/beta 3 complex found on the surface of platelets. This complex is a receptor for fibrinogen and other ligands when platelets are activated. Because the alpha IIb gene is specifically expressed i n megakaryocytes, the 5'-flanking region was studied as a potential mo del for megakaryocyte-specific gene expression, Previous studies have defined some of the important regulatory elements in the 5'-flanking r egion of this gene, The present studies focus on the issue of the mole cular basis by which this TATA-less gene is properly transcribed. A GA -rich region centered 14 bp upstream from the transcriptional start si te appears to be a nonconsensus Sp1-binding site. Binding to this site is of low affinity, but is markedly improved by interaction with prot ein(s) binding at an Ets-consensus site similar to 20 bp further upstr eam. Mutation of the Ets site greatly reduces the ability of Spl to bi nd to its site. Trans-acting nuclear factors binding to and interactio n of the proteins at these two sites have direct effects on the observ ed promoter activity in primary megakaryocyte transient expression stu dies, These studies provide further evidence of the role of interactio ns between Ets-like proteins and Sp1 in transcriptional activation whe n a TATA box is not present in the promoter region of a gene. Based on the presented studies and previous results, a model is proposed for t he regulation of expression of the alpha IIb gene. (C) 1996 by The Ame rican Society of Hematology.