Structural studies of the Ro ribonucleoprotein and the metalloregulator CsoR

Ro ribonucleoproteins are antigenic protein-RNA particles that are the major targets of the immune reaction in autoimmune disorders like systemic lupus erythematosus. The Ro protein has been implicated in cellular RNA quality control, due to its preference for binding misfolded non-coding RNAs such as pre5S ribosomal RNAs and U2 small-nuclear RNAs besides binding cytoplasmic RNAs called Y RNAs. Although well characterized in eukaryotes, an understanding of Ro in prokaryotes is lacking. To gain structural insight into Ro-RNA interactions we have determined a high resolution crystal structure of Rsr, a Ro ortholog from the bacterium D. radiodurans. The structure of Rsr reveals two domains- a flexible, RNA binding HEAT repeat domain and a cation binding vonWillebrand factor A domain. Structural differences between Rsr and Xenopus laevis Ro at the misfolded non-coding RNA binding site suggest a possible conformational switch in Ro that might enable RNA binding. Structural and biochemical characterization reveals that Ro binds cytoplasmic small RNAs called Y RNAs with low nanomolar affinity, to form ~700kDa multimers. Formation of these multimers suggests one possible mode by which Ro RNAs may be targeted towards downstream processing events. Metal responsive transcriptional regulators sense specific metals in the cells and regulate the expression of specific operons involved in export, import or sequestration of the metal. CsoR is a copper(I) specific transcriptional regulator of the cso operon which consists of a putative copper export pump, CtpV. In copper limiting conditions, CsoR binds the operator/promoter region of the cso operon. In increased concentrations of copper (I), CsoR binds copper (I) with high affinity and is released from the operator/promoter site, causing derepression of the cso operon. To gain structural insight into CsoR function, we have solved the crystal structure of copper(I) bound CsoR. The structure reveals a homodimer with a subunit bridging copper site. The trigonal planar geometry and the presence of cysteine and histidine ligands at the metal site are favorable for copper(I) binding. The structure reveals a novel DNA binding fold in CsoR, making it the founding member of a new structural class of metalloregulators.