Liver X receptor beta with mutations in the activation function-2 region is excluded from the nucleolus

Liver X receptors (LXRs) alpha and beta are ligand-induced transcription factors that regulate transcription of genes encoding key regulators of cholesterol metabolism and transport, and of lipogenesis. Despite their high similarity, LXRalpha is the functionally dominant LXR isotype in the liver. The function of nuclear proteins can be affected by their sequestration in the nucleoli. Whereas most nuclear receptors are excluded from the nucleolus, some are not. To explore nucleolar exclusion of LXRalpha and LXRbeta, we used cells expressing cyan fluorescent protein (CFP) chimeras with LXRalpha (CFP-LXRalpha) and wild-type and mutant CFP-LXRbeta and marked the nucleolus with anti-fibrillarin antibody. Significantly more CFP-LXRbeta than CFP-LXRalpha in the nucleoli. Mutations in basic-rich sequences in the DNA binding domain caused some exclusion of CFP-LXRbeta from the nucleolus. Moreover, mutations in the activation function-2, an important protein-protein interaction site in all nuclear receptors, resulted in exclusion of CFP-LXRbeta from the nucleolus. These data suggest protein-protein interactions that may regulate nucleolar sequestration of LXRbeta.