Cysteine modification of a specific repressor protein controls the translational status of nucleus-encoded LHCII mRNAs in Chlamydomonas

Abstract

The cytosolic RNA-binding protein NAB1 represses translation of LHCII (light-harvesting complex of photosystem II) encoding mRNAs by sequestration into translationally silent mRNP complexes in the green alga Chlamydomonas reinhardtii. NAB1 contains 2 cysteine residues, Cys-181 and Cys-226, within its C-terminal RRM motif. Modification of these cysteines either by oxidation or by alkylation in vitro was accompanied by a decrease in RNAbinding affinity for the target mRNA sequence. To confirm the relevance of reversible NAB1 cysteine oxidation for the regulation of its activity in vivo, we replaced both cysteines with serines. All examined cysteine single and double mutants exhibited a reduced antenna at PSII caused by a perturbed NAB1 deactivation mechanism, with double mutations and Cys-226 single mutations causing a stronger and more distinctive phenotype compared with the Cys-181 mutation. Our data indicated that the responsible redox control mechanism is mediated by modification of single cysteines. Polysome analyses and RNA co-immunoprecipitation experiments demonstrated the interconnection of the NAB1 thiol state and its activity as a translation repressor in vivo. NAB1 is fully active in its dithiol state and is reversibly deactivated by modification of its cysteines. In summary, this work is an example that cytosolic translation of nucleus encoded photosynthetic genes is regulated via a reversible cysteine-based redox switch in a RNA-binding translation repressor protein.Chlamydomonas reinhardtii ͉ light harvesting antenna ͉ redox control ͉ translation control T o compensate for changes in light intensity or spectral quality, plants have developed several short-term and longterm mechanisms to regulate the amount of light that is captured by each photosystem (1). One important long-term adaptation strategy of plant organisms involves the complex expression regulation of various nuclear-encoded light harvesting complex (Lhcb) genes (1). All levels of LHCII gene expression are targeted by regulation mechanisms (2-5) which rely on a complex retrograde and anterograde communication between plastid, nucleus, and cytosol (6). The cytosolic translation repressor NAB1, which was identified in a Chlamydomonas reinhardtii light acclimation mutant (4), is the center of interest within this work. NAB1 harbors 2 RNA-binding motifs and 1 of these motifs, located at the N terminus, belongs to the highly conserved family of CSD (cold shock domain) domains. Proteins containing a CSD motif are referred to as Y-box proteins and eukaryotic members of this large family generally contain a second auxiliary RNA-binding domain, which modulates the RNA affinity of the protein but can be dispensable for selective RNA recognition (7). In the case of NAB1, the CSD motif is combined with a C-terminal RRM (RNA recognition motif) domain, which was demonstrated not to be essential for selective RNA recognition (4). It was shown that NAB1 binds to the mRNA of LHCBM (major light-harvesting complex of photosystem II) genes, thereby preventi...

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