In flowering plant plastids and mitochondria, multiple organellar RNA editing
factor (MORF/RIP) proteins are required at most sites for efficient C to U RNA
editing catalyzed by the RNA editosome. MORF proteins harbor a conserved
stretch of residues (MORF-box), form homo- and heteromers and interact with
selected PPR (pentatricopeptide repeat) proteins, which recognize each editing
site. The molecular function of the MORF-box remains elusive since it shares
no sequence similarity with known domains. We determined structures of the A.
thaliana mitochondrial MORF1 and chloroplast MORF9 MORF-boxes which both adopt
a novel globular fold (MORF domain). Our structures state a paradigmatic model
for MORF domains and their specific dimerization via a hydrophobic interface.
We cross-validate the interface by yeast two-hybrid studies and pulldown
assays employing structure-based mutants. We find a structural similarity of
the MORF domain to an N-terminal ferredoxin-like domain (NFLD), which confers
RNA substrate positioning in bacterial 4-thio-uracil tRNA synthetases,
implying direct RNA contacts of MORF proteins during RNA editing. With the
MORF1 and MORF9 structures we elucidate a yet unknown fold, corroborate MORF
interaction studies, validate the mechanism of MORF multimerization by
structure-based mutants and pave the way towards a complete structural
characterization of the plant RNA editosome