MOLECULAR-CLONING AND CHARACTERIZATION OF A NOVEL MOUSE MACROPHAGE GENE THAT ENCODES A NUCLEAR-PROTEIN COMPRISING POLYGLUTAMINE REPEATS ANDINTERSPERSING HISTIDINES

Simple tandem repeats of the trinucleotide sequence CAG encode homopol ymeric stretches of glutamine. Although polyglutamine has been identif ied in diverse proteins, it is present predominantly in transcription factors, We observed that oncogene-immortalized mouse macrophages expr ess several genes that contain a CAG repeat motif. Therefore, we attem pted to clone a novel gene that contains a CAG repeat and is associate d with cytokine activation of macrophages. Screening of a mouse macrop hage cDNA library with a probe comprising 12 consecutive CAG triplets identified at least one unique clone. The cDNA encodes a protein (name d GRP-1 or glutamine repeat protein-1) with 171 amino acids, a calcula ted molecular mass of 21.6 kDa, and a predicted pI of 10.67. Greater t han two-thirds of GRP-1 are only two amino acids, namely glutamine (50 %) and histidine (18%). There are four polyglutamine motifs interspers ed with histidine-rich regions. There is also a putative nuclear local ization signal flanked by sites for possible serine phosphorylation. G RP-1 mRNA was expressed constitutively in some macrophage cell lines a nd B and T cell lines. Interferon-gamma or lipopolysaccharide augmente d GRP-1 mRNA expression in the mouse macrophage cell line ANA-1. Weste rn blot analyses using an antipeptide serum revealed that GRP-1 was lo calized in the nucleus of ANA-1 macrophages and transfected 3T3 fibrob lasts. Overexpression of GRP-1 decreased Sp1-driven chloramphenicol ac etyltransferase gene expression in transient cotransfection experiment s. Because polyglutamine motifs can cause protein oligomerization and can function as transcriptional activation domains, we suggest that GR P-1 may be a transcription factor associated with interferon-gamma- or lipopolysaccharide-induced activation of macrophages.