Members of the tristetraprolin family of tandem CCCH zinc finger proteins exhibit CRM1-dependent nucleocytoplasmic shuttling

Abstract

Members of the tristetraprolin (TTP) family of CCCH tandem zinc finger proteins can bind directly to certain types of AU-rich elements (AREs) in mRNA. Experiments in TTP-deficient mice have shown that TTP is involved in the physiological destabilization of at least two cytokine mRNAs, those encoding tumor necrosis factor α and granulocyte-macrophage colony-stimulating factor. The two other known mammalian members of the TTP family, CMG1 and TIS11D, also contain ARE-binding CCCH tandem zinc finger domains and can also destabilize ARE-containing mRNAs. To investigate the effects of primary sequence on the subcellular localization of these proteins, we constructed green fluorescent protein fusions with TTP, CMG1, and TIS11D; these were predominantly cytoplasmic when expressed in 293 or HeLa cells. Deletion and mutation analyses revealed functional nuclear export signals in the amino terminus of TTP and in the carboxyl termini of CMG1 and TIS11D. This type of leucine-rich nuclear export signal interacts with the nuclear export receptor CRM1; abrogation of CRM1 activity resulted in nuclear accumulation of TTP, CMG1, and TIS11D. These proteins are thus nucleocytoplasmic shuttling proteins and rely on CRM1 for their export from the nucleus. Although TTP, CMG1, and TIS11D lack known nuclear import sequences, mapping experiments revealed that their nuclear accumulation required an intact tandem zinc finger domain but did not require RNA binding ability. These findings suggest possible roles for nuclear import and export in the regulation of cellular TTP, CMG1, and TIS11D activity.