Involvement of Arabidopsis RACK1 in Protein Translation and Its Regulation by Abscisic Acid      

Abstract

Earlier studies have shown that RACK1 functions as a negative regulator of abscisic acid (ABA) responses in Arabidopsis (Arabidopsis thaliana), but the molecular mechanism of the action of RACK1 in these processes remains elusive. Global gene expression profiling revealed that approximately 40% of the genes affected by ABA treatment were affected in a similar manner by the rack1 mutation, supporting the view that RACK1 is an important regulator of ABA responses. On the other hand, coexpression analysis revealed that more than 80% of the genes coexpressed with RACK1 encode ribosome proteins, implying a close relationship between RACK1's function and the ribosome complex. These results implied that the regulatory role for RACK1 in ABA responses may be partially due to its putative function in protein translation, which is one of the major cellular processes that mammalian and Saccharomyces cerevisiae RACK1 is involved in. Consistently, all three Arabidopsis RACK1 homologous genes, namely RACK1A, RACK1B, and RACK1C, complemented the growth defects of the S. cerevisiae cross pathway control2/rack1 mutant. In addition, RACK1 physically interacts with Arabidopsis Eukaryotic Initiation Factor6 (eIF6), whose mammalian homolog is a key regulator of 80S ribosome assembly. Moreover, rack1 mutants displayed hypersensitivity to anisomycin, an inhibitor of protein translation, and displayed characteristics of impaired 80S functional ribosome assembly and 60S ribosomal subunit biogenesis in a ribosome profiling assay. Gene expression analysis revealed that ABA inhibits the expression of both RACK1 and eIF6. Taken together, these results suggest that RACK1 may be required for normal production of 60S and 80S ribosomes and that its action in these processes may be regulated by ABA.

New Evidence for Differential Roles of L10 Ribosomal Proteins from Arabidopsis

“…Thus, Arabidopsis RPL10 proteins are functionally equivalent to the RPL10 from S. cerevisiae and, therefore, these proteins are interchangeable. Similarly, the three Arabidopsis RACK1 proteins, which are components of the 40S ribosome subunit, complement S. cerevisiae cross pathway control2/rack1 mutants, as was shown previously for the mammalian RACK1 protein (Gerbasi et al, 2004;Guo et al, 2011). However, although all three Arabidopsis genes are functional in rescuing yeast lethality, slower growth rates were observed when compared with that obtained using the endogenous yeast protein, indicating differences in the complementation efficiency.…”

“…Furthermore, it has been recently demonstrated that ribosomal proteins control auxinmediated developmental programs by translational regulation of auxin response factors (Rosado et al, 2012). In addition, the characterization of single, double, and, in certain cases, triple mutants as well as complementation by paralog genes have demonstrated full, partial, and no redundancy between members of ribosomal protein families (Briggs et al, 2006;Guo and Chen, 2008;Guo et al, 2011;Horiguchi et al, 2011;Stirnberg et al, 2012).…”

Combined Large-Scale Phenotyping and Transcriptomics in Maize Reveals a Robust Growth Regulatory Network

“…The genes within the two subnetworks were highly connected to each other and to a limited set of other genes. For subnetwork 1, one of these other genes, GRMZM2G038032, shows homology to Arabidopsis RECEPTOR FOR ACTIVATED C PROTEIN KINASE1 (RACK1) genes involved in ribosome biogenesis (Guo et al, 2011); it was shown that lossof-function mutations in RACK1 genes in Arabidopsis severely affect rosette leaf production and root growth (Guo and Chen, 2008). The ribosomal genes in this first subnetwork were also highly connected to GRMZM2G067877, a homolog of an Arabidopsis mitochondrial adenine nucleotide transporter (ADNT1) that plays a role in energy supply in heterotrophic tissues.…”

Translational control by 80S formation and 60S availability: The central role of eIF6, a rate limiting factor in cell cycle progression and tumorigenesis

“…5 eIF6 binds RACK1 5,23 that resides on 48S subunits. 24 This model places eIF6 activity downstream of the growth factor/PKC signaling cascade and explains observations indicating that eIF6 can repress translation in vitro but not in vivo when ras/PKC stimulation is performed (see below).…”