The regulated expression of a TATA-less, platelet-specific gene, alphaIIb

The megakaryocyte (MK)-specific integrin, alphaIIb, is the alpha-subunit of the alphaIIb/beta3 complex found on the surface of platelets. This complex is a receptor for fibrinogen and other ligands when platelets are activated. Because the alphaIIb gene is specifically expressed in MKs, this gene was studied as a potential model for MK-specific gene expression. Previous studies have defined some of the important regulatory elements in 912 bp of the immediate 5'-flanking region of this gene. These studies defined several important elements including two GATA-binding elements and an Ets-binding element. Using a primary rat marrow expression system, we demonstrated that one of the GATA-binding elements, -454 bp upstream of the transcriptional start site (GATA454), is critical for expression of the alphaIIb gene. A potential negative regulatory element was found between -100 and -200 bp upstream of both the rat and human alphaIIb genes. The biological basis by which this negative regulatory region effects expression is not well understood. Recent studies have focused on the issue of the molecular basis by which this TATA-less gene is properly transcribed. We found that a GA-rich region approximately 14 bp upstream from the transcriptional start site appears to be a nonconsensus Sp1-binding site that interacts with an Ets-consensus site approximately 20 bp further upstream. These studies provide further evidence of the role of interactions between Ets-like proteins and Sp1 in transcriptional activation when 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 the regulation of expression of the alphaIIb gene. In studies looking at more distal regulatory elements, we have found, using the primary rat marrow expression system, that 2.9 kb of 5'-flanking alphaIIb sequence has as high a level of expression as the 912 bp construct. Whether either of these lengths of 5'-flanking region can result in tissue-specific expression in transgenic models is presently being investigated. In addition, while a published report suggests that the two genes alphaIIb and beta3 are physically linked within a 250 kb region of genomic DNA, analysis of yeast artificial chromosome clones and genomic pulsed field gel electrophoresis analysis are consistent with these two genes not being tightly linked and being >1 mb apart, suggesting that these two genes do not form a single, tissue-specific locus.

Characterization of cDNAs encoding the p44 and p35 subunits of human translation initiation factor eIF3

Eukaryotic translation initiation factor 3 (eIF3) is a large multisubunit complex that plays a central role in the initiation of translation. It binds to 40 S ribosomal subunits resulting in dissociation of 80 S ribosomes, stabilizes initiator methionyl-tRNA binding to 40 S subunits, and is required for mRNA binding. eIF3 has an aggregate molecular mass of approximately 600 kDa and comprises at least 10 subunits. The cDNAs encoding eight of the subunits have been cloned previously (p170, p116, p110, p66, p48, p47, p40, and p36). Here we report the cloning and characterization of human cDNAs encoding two more subunits of human eIF3, namely eIF3-p44 and eIF3-p35. These proteins are immunoprecipitated by affinity-purified anti-eIF3-p170 antibodies, indicating they are components of the eIF3 complex. Far Western analysis shows that eIF3-p44 interacts strongly and specifically with the eIF3-p170 subunit, and weakly with p116/p110, p66, p40, and itself. eIF3-p44 contains an RNA recognition motif near its C terminus. Northwestern blotting shows that eIF3-p44 binds 18 S rRNA and beta-globin mRNA. Possession of cloned cDNAs encoding all 10 subunits of eIF3 provides the tools necessary to elucidate the functions of the individual subunits and the structure of the eIF3 complex.

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 of 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 in megakaryocytes, the 5'-flanking region was studied as a potential model for megakaryocyte-specific gene expression. Previous studies have defined some of the important regulatory elements in the 5'-flanking region of this gene. The present studies focus on the issue of the molecular basis by which this TATA-less gene is properly transcribed. A GA-rich region centered 14 bp upstream from the transcriptional start site appears to be a nonconsensus Sp1-binding site. Binding to this site is of low affinity, but is markedly improved by interaction with protein(s) binding at an Ets-consensus site approximately 20 bp further upstream. Mutation of the Ets site greatly reduces the ability of Sp1 to bind to its site. Trans-acting nuclear factors binding to and interaction of the proteins at these two sites have direct effects on the observed promoter activity in primary megakaryocyte transient expression studies. These studies provide further evidence of the role of interactions between Ets-like proteins and Sp1 in transcriptional activation when 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 the regulation of expression of the alpha IIb gene.

Platelet glycoprotein IIb gene expression as a model of megakaryocyte-specific expression

Glycoprotein (GP)IIb/IIIa is an integrin complex normally restricted in its expression to platelets and the megakaryocytes from which they are derived. This complex functions as a receptor for fibrinogen and other ligands and is involved in platelet aggregation. The receptor complex is expressed at high levels during final megakaryocyte differentiation. Further, while GPIIIa is expressed in other tissues as part of the vitronectin receptor, GPIIb is only expressed on maturing megakaryocytes and the platelets derived from them. Thus studies of the GPIIb gene may serve as a model of gene regulation during this process. Over the past several years, the genes for both GPIIb and IIIa have been cloned and analyzed. The GPIIb gene contains 30 exons over 18 kilobases (kb). The transcriptional start site has been determined and there does not appear to be a TATA-box immediately upstream of this site. Studies have been done to define regulatory elements upstream of the transcriptional start site. Most of these studies focused on the human promoter and on studies using megakaryocytic cell lines. These studies have defined several important tissue-specific promoter elements including a GATA454 site (454 basepairs upstream of the transcriptional start site that involves a GATA-binding consensus sequence), a GATA54 site and an Ets35 site (that involves an Ets-binding consensus sequence). Expression studies with megakaryocytic cell lines suggest that each of these sites effects expression approximately threefold. Further, an Ets510 site was also described that had a similar effect. While these studies were underway, we pursued studies of the rat 5'-flanking region using a rat primary marrow expression system. Qualitatively, our data support the human data; however, quantitatively, we found significant differences from the human studies done in cell lines. We found that the major tissue-specific promoter element was the GATA454 site. Mutations altering this site result in an approximately fiftyfold drop in expression. In comparison, eliminating the Ets510 site by truncation or point mutation had only a twofold effect on expression. Mutations at the Ets35 site did effect expression at a high level, decreasing expression approximately fifteenfold, while mutations at the GATA54 site effected expression by approximately ninefold. In addition, using 50 bp deletions, we have preliminarily defined two domains from -450 to -351 bp and -150 to -101 bp upstream of the transcriptional start site that effected expression. The former appears to contain a positive regulatory element, while the latter appears to be a silencer element.(ABSTRACT TRUNCATED AT 400 WORDS)

Characterization of regulatory elements in the 5'-flanking region of the rat GPIIb gene by studies in a primary rat marrow culture system

Glycoprotein (GP)IIb/IIIa, an integrin complex found on the surface of platelets, is a receptor for fibrinogen and other ligands, and is involved in platelet aggregation. Because GPIIb is specifically expressed in megakaryocytes, we have studied the 5'-flanking region of the rat (r) GPIIb gene as a model of a megakaryocyte-specific gene. The studies presented here used a rat marrow expression system, which allows the study of primary cells undergoing terminal differentiation into megakaryocytes. The determination of megakaryocyte-specific expression of DNA constructs was possible by immunomagnetically separating megakaryocytes from total bone marrow cells. Transient expression constructs, containing varying lengths of the 5'-flanking region from -39 to -912 bp, localized a regulatory element between -460 and -439 bp upstream of the transcriptional start site. This region contains a GATA consensus binding element between -457 and -454 (GATA454). Further constructs demonstrated that this GATA binding element was indeed essential for expression. A 25-bp substitution, covering the region -450 to -426 immediately downstream of the GATA454, demonstrated that this region was essential for full expression, which suggests that this region may interact with the GATA454 site in promoting high-level lineage-specific expression. To define regulatory elements between the GATA454 and the transcriptional start site further, we tested additional constructs derived from the original -912 construct; each of which contained the GATA454 but had different 50-bp deletions from -450 to the start site. Virtually all of these constructs continued to show high-level tissue-specific expression. The deleted -150 to -101 construct had twice the level of expression of the full-length wild-type construct; therefore, this region may contain a negative regulatory element. Comparison of our data with expression studies performed with the 5'-region of the human GPIIb gene using HEL cells, a cell line with some megakaryocytic properties, demonstrates significant differences, which may reflect our use of primary rate bone marrow cells. In particular, our study points to the importance of the GATA454 for high levels of GPIIb expression in developing megakaryocytes.