Mapping of a minimal AU-rich sequence required for lipopolysaccharide-induced binding of a 55-kDa protein on tumor necrosis factor-alpha mRNA.

In monocyte/macrophage cells, the translation of tumor necrosis factor-alpha (TNF-alpha) mRNA is tightly controlled. In unstimulated cells, TNF-alpha mRNA is translationally repressed. However, upon stimulation of the cells with various agents (e.g. lipopolysaccharides (LPS) and viruses), this repression is overcome and translation occurs. The key element in this regulation is the AU-rich sequence present in the 3'-untranslated region of TNF-alpha mRNA. Several groups have described the binding of proteins on AU-rich elements (AREs). We have previously reported the binding of two cytosolic protein complexes (1 and 2) to the TNF-alpha mRNA ARE, one of which (complex 2) is observed only following induction of TNF-alpha production by LPS. In this report, we have demonstrated that complex 1 involves a long fragment of the ARE, whereas the formation of the LPS-inducible complex 2 requires a minimal sequence which corresponds to the nonanucleotide UUAUUUAUU. Furthermore, we show that the RNA-binding protein involved in complex 2 has an apparent molecular mass of 55 kDa. Finally, we tested other AREs for their ability to form complex 2. We observed that the ARE derived from granulocyte/macrophage colony-stimulating factor mRNA, which does contain the nonanucleotide, is able to sustain the LPS-induced binding of the 55-kDa protein. However, c-myc mRNA, which does not contain the nonanucleotide, is unable to promote the formation of any LPS-induced complex.