Crystallographic studies on UNR, a protein important for translation and decay of mRNA, and on the RNA-induced silencing complex RISC

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The RNA-binding protein UNR is involved in the IRES-mediated initiation of mRNA translation, the mCRD-dependent decay of mRNA, and in the inhibition of msl2-mRNA translation as co-repressor of the sex-lethal protein. UNR binds directly to the IRES- or the mCRD-element of mRNAs, and UNR is also capable to form RNA-free complexes with other proteins like the Poly(A)-binding protein, the UNR-interacting protein (UNRIP), and the sex-lethal protein. UNR consists of five cold shock domains, but their role with respect to the different functional complexes has to be elucidated. In order to understand the structural basis for the versatile functions of UNR we want to determine the three-dimensional structure of UNR and of functional UNR-containing proteinprotein or protein-RNA complexes by means of X-ray crystallography. A second project concerns the preparation, crystallization and crystal structure determination of an RNA-induced silencing complex (RISC), which plays a central role in the control of gene expression by RNA interference. A minimal human RISC, formed by the proteins Dicer, Argonaute2 and TRBP, was shown to exhibit dsRNA loading, strand selection, and target mRNA cleavage activities. We will establish a eukaryotic co-expression system in order to obtain reasonable amounts of the heterotrimeric protein complex for crystallization experiments. Major aim is the crystal structure analysis of the minimal RISC in different functional states. Additionally, this recombinant minimal RISC will be used for single particle cryo-electron microscopy studies.