Identification of the sequence determinants mediating the nucleo-cytoplasmic shuttling of TIAR and TIA-1 RNA-binding proteins.

TIAR and TIA-1 are two closely related RNA-binding proteins which possess three RNA recognition motifs (RRMs) followed by an auxiliary region. These proteins are involved in several mechanisms of RNA metabolism, including alternative hnRNA splicing and regulation of mRNA translation. Here we characterize the subcellular localization of these proteins in somatic cells. We demonstrate that TIAR and TIA-1 continuously shuttle between the cytoplasm and the nucleus and belong to the class of RNA-binding proteins whose nuclear import is transcription-dependent. We identified RRM2 and the first half of the auxiliary region as important determinants for TIAR and TIA-1 nuclear accumulation. In contrast, the nuclear export of TIAR and TIA-1 is mediated by RRM3. Both RRMs contribute to TIAR and TIA-1 nuclear accumulation or export by their RNA-binding capacity. Indeed, whereas mutations of the highly conserved RNP2 or RNP1 peptides in RRM2 redistribute TIAR to the cytoplasm, similar modifications in RRM3 abolish TIAR nuclear export. Moreover, TIAR and TIA-1 nuclear accumulation is a Ran-GTP-dependent pathway, in contrast to its nuclear export which is unaffected by Ran-GTP depletion and which is independent of the major CRM1-exporting pathway. This study demonstrates the importance of TIAR and TIA-1 RNA-binding domains for their subcellular localization and provides the first evidence for distinct functions of TIAR and TIA-1 RRMs.