Signal Sequences That Target Nuclear Import and Nuclear Export of Pre-mRNA-binding Proteins

Abstract

Most nuclear proteins are targeted to the nucleus by virtue of nuclear localization signal (NLS) sequences. The identification of NLSs has led to the isolation and characterization of many factors involved in nuclear protein import, a reaction that can now be reproduced in vitro from purified components (Gröich et al. 1994; for review, see Fabre and Hurt 1994; Moore and Blobel 1994). We have been studying the intracellular transport of a group of nuclear pre-mRNA-binding proteins, collectively termed hnRNP proteins, whose transport properties appear to, at least in part, differ from those of the majority of nuclear proteins (Dreyfuss et al. 1993; Piñol-Roma and Dreyfuss 1993). hnRNP proteins, designated A1-U, associate with nascent RNA polymerase II (pol II) transcripts to form hnRNP complexes, which serve as the substrates for the RNA processing reactions that occur to produce mature, translatable mRNA molecules. A subset of these proteins are removed from the...