A yeast RNA-binding protein shuttles between the nucleus and the cytoplasm.

Abstract

RNA-binding proteins have been suggested to move in association with RNA as it leaves the nucleus. The NPL3 gene of the yeast Saccharomyces cerevisiae encodes a nuclear protein with consensus RNA-binding motifs and similarity to heterogeneous nuclear ribonucleoproteins and members of the S/R protein family. We show that although Npl3 is located in the nucleus, it can shuttle between nuclei in yeast heterokaryons. In contrast, other nucleus-targeted proteins do not leave the nucleus under similar conditions. Mutants missing the RNA-binding motifs or the N terminus are still capable of shuttling in and out of the nucleus. Npl3 mutants missing the C terminus fail to localize to the nucleus. Overproduction of Npl3 in wild-type cells slows cell growth. This toxicity depends on the presence of a series of unique repeats in the N terminus and localization to the nucleus. We suggest that the properties of Npl3 are consistent with it being involved in export of RNAs from the nucleus.Proteins and RNAs are continuously moving in and out of the nucleus via the nuclear pore. Proteins destined for the nucleus often contain short stretches of amino acids, termed nuclear localization sequences (NLSs), that can direct them to the nucleus (15,17,34). Other proteins may enter the nucleus by associating with an NLS-bearing protein (38). Once at the pore, the protein to be imported binds and is transported in a reaction that requires ATP (26, 28) and proteins of the nuclear pore complex (12,13,24,32). Much less is known about exit of macromolecules from the nucleus. All classes of RNAs are thought to be exported to the cytoplasm as RNA-protein complexes via the nuclear pores (10). Most likely this is also an active process that may involve specific targeting of macromolecules. Experiments with microinjected RNAs indicate that exit of RNA from the nucleus is an energy-dependent, facilitated, and saturable process (7,37).Some nuclear proteins, in particular those that bind RNA, repeatedly shuttle in and out of the nucleus (9). The functional significance of this export-reimport is still unclear. However, one proposal is that nuclear shuttling proteins may be directly involved in nucleocytoplasmic exchange of macromolecules. By one model, certain RNA-binding proteins would be part of the export substrate that moves with the RNA from the nucleus via the pore. Following release of their RNA "cargo" by exchange with cytoplasmic RNA-binding proteins, the proteins that accompanied RNA out would rapidly reenter the nucleus. Implicit in this model is the idea that some proteins would contain information that not only targets them to the

Arginine methylation facilitates the nuclear export of hnRNP proteins

“…8B). All three factors are known to exit the nucleus and to bind pre-mRNAs (Flach et al 1994;Gorlich et al 1996;Lee et al 1996;Kessler et al 1997), thus supporting the possibility that they could be actively involved in export. The presence of proteins associated with capping and polyadenylation could lead to more efficient packaging and export of the RNA to the cytoplasm.…”

“…We have shown previously that both Npl3p and Hrp1p exit the nucleus (Flach et al 1994;Lee et al 1996;Kessler et al 1997). To test whether methylation of Npl3p and Hrp1p has an effect on their export, we utilized a nuclear export assay (Lee et al 1996).…”

“…These include two RNPtype RNA-recognition motifs (RRMs), and a glycine-rich domain in the form of 15 arginine-glycine-glycine (RGG) repeats (Birney et al 1993). Although Npl3p appears to localize to the nucleus in a distinct non-nucleolar pattern similar to human hnRNPs (Flach et al 1994), it actually shuttles between the nucleus and the cyto- Cold Spring Harbor Laboratory Press on May 8, 2018 -Published by genesdev.cshlp.org Downloaded from …”

“…These include two RNPtype RNA-recognition motifs (RRMs), and a glycine-rich domain in the form of 15 arginine-glycine-glycine (RGG) repeats (Birney et al 1993). Although Npl3p appears to localize to the nucleus in a distinct non-nucleolar pattern similar to human hnRNPs (Flach et al 1994), it actually shuttles between the nucleus and the cyto-plasm (Flach et al 1994;Lee et al 1996). Strains bearing mutant alleles of NPL3 accumulate poly(A) + RNA in the nucleus (Russell and Tollervey 1992;Singleton et al 1995;Lee et al 1996).…”

A specific subset of SR proteins shuttles continuously between the nucleus and the cytoplasm

“…These include nucleolar proteins, such as NO38, nucleolin, and Nopp140 (Borer et al 1989;Meier and Blobel 1992), steroid hormone receptors, such as the progesterone receptor (Guiochon-Mantel et al 1991), the U1 snRNP-specific polypeptide U1A (Kambach and Mattaj 1992), heat shock-related proteins (Mandell and Feldherr 1990), the HIV-1 Rev protein (Meyer and Malim 1994), and the Saccharomyces cerevisiae protein Npl3p (Flach et al 1994;Lee et al 1996). Although the precise functions of nucleocytoplasmic shuttling are not understood, this process may reflect the role of shuttling proteins as carriers for the delivery of protein or RNA molecules to the nucleus and cytoplasm.…”