A Unique mRNA Initiated within a Middle Intron of WHSC1/MMSET Encodes a DNA Binding Protein That Suppresses HumanIL-5 Transcription

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Abstract
Human interleukin (IL)-5 gene transcription is regulated by several transcription factor binding sites, including CLE 0, GATA, and a region from position -123 to -92 known as response element (RE)-II. By expression cloning, a partial protein was identified that bound to concatamers of RE-II. Recombinant protein derived from this initial complementary DNA (cDNA) encoding the partial protein specifically bound to RE-II-containing oligonucleotides in electromobility shift assays. The complete sequence (3,649 bp) was determined by 5' rapid amplification of cDNA ends and comparisons to existing ESTs, and found to be identical to the 3' half of Wolf-Hirschhorn syndrome candidate 1, (WHSC1; also known as Multiple Myeloma SET domain [MMSET]). The full-length protein contains an SET domain and two plant homeodomain-type zinc fingers. Transcription initiation of RE-II binding protein (RE-IIBP) messenger RNA (mRNA) uniquely occurred within the middle of WHSC1 near a region that exhibits complex mRNA splicing. RE-IIBP reactive polyclonal antisera identified proteins in human T-cell nuclear protein extracts of 62 and 66 kD that were consistent with the length of the longest open reading frame in RE-IIBP. In contrast, WHSC1 is predicted to encode a protein of 136 kD. In activated human Jurkat and murine D10.G4.1 T cells, expression of full-length and truncated forms of RE-IIBP repressed RE-II promoter activity of a 5X-RE-II luciferase reporter construct by as much as 75%. In addition, RE-IIBP expressed in activated D10.G4.1 T cells inhibited endogenous murine IL-5 production. The repressor activity of RE-IIBP is consistent with the presence of an SET domain that is found in other proteins that act as gene silencers.