Functional dissection of hnRNP D suggests that nuclear import is required before hnRNP D can modulate mRNA turnover in the cytoplasm

Many shuttling proteins not only function in the nucleus but also control mRNA fates in the cytoplasm. We test whether a link exists between their nuclear association with mRNPs and their cytoplasmic functions using the p37 isoform of hnRNP D, which inhibits the rapid cytoplasmic mRNA decay in NIH3T3 cells. We showed that p37 shuttles between nucleus and cytoplasm, and narrowed down the nuclear import signal to a 50-amino-acid C-terminal domain. A p37 mutant missing this domain, still capable of associating with target mRNAs in vitro, was confined to the cytoplasm, where it was unable to block cytoplasmic mRNA turnover. Introducing heterologous shuttling domains to this mutant, thereby restoring its ability to enter the nucleus, concomitantly restored its cytoplasmic function. Association of p37 with its target mRNAs can only be detected when it can enter the nucleus. Our results suggest that nuclear import of hnRNP D is a prerequisite for it to exert its cytoplasmic function. This study provides a useful model system to elucidate the mechanisms by which "nuclear history" affects cytoplasmic mRNA fates.