RACK1 function in the miRNA pathway

MicroRNAs (miRNAs) regulate different aspects of plant development by post-transcriptional regulation of target genes. In Arabidopsis, DICER-LIKE 1 (DCL1) processes miRNA precursors (pri-miRNAs) to miRNA duplexes, which associate with ARGONAUTE 1 (AGO1). AGO1 together with the miRNA guide strand binds complementary RNA sequences within target mRNAs. Additional proteins act in concert with DCL1 (e.g. HYL1 and SERRATE) and AGO1, respectively, to facilitate efficient and precise pri-miRNA processing and loading into the effector protein. Here, we show that RECEPTOR OF ACTIVATED C KINASE 1 (RACK1) is a novel component of the Arabidopsis miRNA pathway. RACK1 is a seven-bladed WD-repeat protein that has previously been shown to act as a scaffold protein mediating multiple simultaneous protein-protein interactions. Our molecular analyses demonstrate that RACK1 function is required for controlling miRNA-mediated gene expression. rack1 mutants contain only low levels of mature miRNAs without affecting the first step of pri-miRNA processing. Physical and genetic interaction studies revealed that RACK1 acts in concert with AGO1 and also interacts with a SERRATE, a component of the miRNA processing machinery. These results suggest that RACK1 also functions as a scaffold protein in the miRNA pathway to orchestrates miRNA maturation steps after the initial events of pri-miRNA processing. sequencing of small RNAs from WT and rack1abc mutants (two biological replicates each)