Nuclear import of dimerized ribosomal protein Rps3 in complex with its chaperone Yar1.

Valentin Mitterer,Dieter Kressler,Ruth Birner-Gruenberger,Helmut Bergler,Nadine Gantenbein,Brigitte Pertschy,Guillaume Murat

doi:10.1038/srep36714

Valentin Mitterer, Dieter Kressler + Show 5 more

Open Access

https://doi.org/10.1038/srep36714

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Abstract

After their cytoplasmic synthesis, ribosomal proteins need to be transported into the nucleus, where they assemble with ribosomal RNA into pre-ribosomal particles. Due to their physicochemical properties, they need protection from aggregation on this path. Newly synthesized ribosomal protein Rps3 forms a dimer that is associated with one molecule of its specific chaperone Yar1. Here we report that redundant pathways contribute to the nuclear import of Rps3, with the classical importin α/β pathway (Kap60/Kap95 in yeast) constituting a main import route. The Kap60/Kap95 heterodimer mediates efficient nuclear import of Rps3 by recognition of an N-terminal monopartite nuclear localization signal (NLS). This Rps3-NLS is located directly adjacent to the Yar1-binding site and, upon binding of Kap60 to Rps3, Yar1 is displaced from the ribosomal protein in vitro. While Yar1 does not directly interact with Kap60 in vitro, affinity purifications of Yar1 and Rps3, however, revealed that Kap60 is present in the Rps3/Yar1 complex in vivo. Indeed we could reconstitute such a protein complex containing Rps3 and both Yar1 and Kap60 in vitro. Our data suggest that binding of Yar1 to one N-domain and binding of Kap60 to the second N-domain of dimerized Rps3 orchestrates import and protection of the ribosomal protein.

Highlights

After their cytoplasmic synthesis, ribosomal proteins need to be transported into the nucleus, where they assemble with ribosomal RNA into pre-ribosomal particles
In contrast to ribosomal RNAs (rRNAs), which is synthesized in the nucleolus, r-proteins are translated in the cytoplasm
In the classical import pathway, importin α(Kap60/Srp[1] in yeast) functions as a transport adaptor that binds to specific nuclear localization signals (NLS) of cargo proteins as well as to importin β1 (Kap[95] in yeast), which interacts with the hydrophobic FG-repeat meshwork[11,12,13,14]

Summary

Introduction

Ribosomal proteins need to be transported into the nucleus, where they assemble with ribosomal RNA into pre-ribosomal particles. In the classical import pathway, importin α(Kap60/Srp[1] in yeast) functions as a transport adaptor that binds to specific nuclear localization signals (NLS) of cargo proteins as well as to importin β1 (Kap[95] in yeast), which interacts with the hydrophobic FG-repeat meshwork[11,12,13,14]. R-proteins are RNA-binding proteins and often contain lysine- and arginine-rich, unfolded extensions that protrude into the interior of the ribosome, where they form critical interactions with rRNA required for the integrity of the ribosomal structure[17]. Most of these assembly chaperones were shown to capture the nascent polypeptide chain of their dedicated r-protein clients in a co-translational manner[23,24]

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