The human IMPs (insulin-like growth factor II mRNA-binding proteins) belong to a vertebrate zipcode-binding protein family consisting of two RNA recognition motifs and four K homology domains and have been implicated in cytoplasmic mRNA localization, turnover and translational control. In the present study, we show that IMP1 is capable of translocating into nuclei of NIH 3T3 fibroblasts and its immunoreactivity is present in the nuclei of human spermatogenic cells. IMP1 does not contain a simple import signal, but nuclear entry was facilitated by disruption of RNA binding and cytoplasmic granule formation. IMP1 contains two NESs (nuclear export signals) within the RNA-binding K homology domains 2 and 4. The former is a leucine-rich leptomycin B-sensitive NES, whereas the latter is a leptomycin B-insensitive NES. Taken together, these results indicate that IMP1 may attach to its target mRNAs in the nucleus and thereby define the cytoplasmic fate of the transcripts.
Abbreviations used: FRAP, fluorescence recovery after photobleaching; GFP, green fluorescent protein; hnRNP, heterogeneous nuclear ribonucleoprotein; IMP, insulin-like growth factor II mRNA-binding protein; KH, K homology; NES, nuclear export signal; NF90, nuclear factor 90; NLS, nuclear localization signal; NPC, nuclear pore complex; RBP, RNA-binding protein; RRM, RNA recognition motif; RT, reverse transcriptase; SV 40, Simian virus 40; TBS, Tris-buffered saline; UTR, untranslated region; YFP, yellow fluorescent protein; ZBP, zipcode-binding protein; for brevity, the one-letter system for amino acids has been used: L318, e.g. means Leu318.
Present address: Molecular Cellular and Developmental Biology, University of Colorado, CB 347, Boulder, CO 80309-0347, U.S.A.