A new miRNA complex assembly partner – Intrinsically disordered but an important scaffold ($) (Plant Physiol)

Gene regulation through microRNAs (miRNA) starts in the nucleus with transcription and processing into mature miRNA duplexes. Loading of miRNA into ARGONAUTE1 (AGO1) and the assembly of the RNA-Induced Silencing Complex was thought to be exclusively a cytoplasmic process, but new research reports this also happens in the nucleus through unknown mechanisms. Tomassi, Re, and colleagues identified CONSTITUTIVE ALTERATIONS IN THE SMALL RNAS PATHWAYS9 (CARP9) as a new partner in the miRNA pathway. CARP9 is an intrinsically disordered protein that localizes to the nucleus; CARP9 paralogs are also predicted to be disordered in all plant species. Mutations in CARP9 produced morphological alterations, a mild reduction in the miRNA accumulation, and impaired gene silencing. The authors discovered that CARP9 interacts with HYPONASTIC LEAVES1 (HYL1) to promote HYL1-AGO1 interactions for loading mature miRNA into AGO1. The data suggest that CARP9 acts as a scaffold for the formation of a nuclear post pri-miRNA processing complex to allow proper loading of miRNA and, regulation of gene expression. (Summary by Katy Dunning @plantmomkaty) Plant Physiology 10.1104/pp.20.00258