Molecular cloning and characterization of a novel mouse macrophage gene that encodes a nuclear protein comprising polyglutamine repeats and interspersing histidines

Simple tandem repeats of the trinucleotide sequence CAG encode homopolymeric stretches of glutamine. Although polyglutamine has been identified in diverse proteins, it is present predominantly in transcription factors. We observed that oncogene-immortalized mouse macrophages express several genes that contain a CAG repeat motif. Therefore, we attempted to clone a novel gene that contains a CAG repeat and is associated with cytokine activation of macrophages. Screening of a mouse macrophage cDNA library with a probe comprising 12 consecutive CAG triplets identified at least one unique clone. The cDNA encodes a protein (named GRP-1 or glutamine repeat protein- l) with 171 amino acids, a calculated molecular mass of 21.6 kDa, and a predicted pI of 10.67. Greater than two-thirds of GRP-1 are only two amino acids, namely glutamine (50%) and histidine (18%). There are four polyglutamine motifs interspersed with histidine-rich regions. There is also a putative nuclear localization signal flanked by sites for possible serine phosphorylation. GRP-1 mRNA was expressed constitutively in some macrophage cell lines and B and T cell lines. Interferon-γ or lipopolysaccharide augmented GRP-1 mRNA expression in the mouse macrophage cell line ANA-1. Western blot analyses using an antipeptide serum revealed that GRP-1 was localized in the nucleus of ANA-1 macrophages and transfected 3T3 fibroblasts. Overexpression of GRP-1 decreased Sp1-driven chloramphenicol acetyltransferase gene expression in transient cotransfection experiments. Because polyglutamine motifs can cause protein oligomerization and can function as transcriptional activation domains, we suggest that GRP-1 may be a transcription factor associated with interferon-γ- or lipopolysaccharide- induced activation of macrophages.