Nonhierarchical ribonucleoprotein assembly suggests a strain-propagation model for protein-facilitated RNA folding.
Proteins play diverse and critical roles in cellular ribonucleoproteins (RNPs) including promoting formation of and stabilizing active RNA conformations. Yet, the conformational changes required to convert large RNAs into active RNPs have proven difficult to characterize fully. Here we use high-resolution approaches to monitor both local nucleotide flexibility and solvent accessibility for nearly all nucleotides in the bI3 group I intron RNP in four assembly states: the free RNA, maturase-bound RNA, Mrs1-bound RNA, and the complete six-component holocomplex. The free RNA is misfolded relative to the secondary structure required for splicing. The maturase and Mrs1 proteins each stabilized long-range tertiary interactions, but neither protein alone induced folding into the functional secondary structure. In contrast, simultaneous binding by both proteins results in large secondary structure rearrangements in the RNA and yielded the catalytically active group I intron structure. Secondary and tertiary folding of the RNA component of the bI3 RNP are thus not independent: RNA folding is strongly nonhierarchical. These results emphasize that protein-mediated stabilization of RNA tertiary interactions functions to pull the secondary structure into an energetically disfavored, but functional, conformation and emphasize a new role for facilitator proteins in RNP assembly.
Department of Chemistry, University of North Carolina, Chapel Hill, North Carolina 27599-3290, USA.
Caia D S
CD
Duncan
Kevin M
KM
Weeks
D004722:Endoribonucleases; D024101:Mitochondrial Proteins; D009690:Nucleic Acid Conformation; D009713:Nucleotidyltransferases; D011485:Protein Binding; D012313:RNA / Q000737:chemistry*; D020871:RNA Stability; D016601:RNA-Binding Proteins; D012261:Ribonucleoproteins / Q000096:biosynthesis* / Q000737:chemistry; D012441:Saccharomyces cerevisiae / Q000235:genetics; D029701:Saccharomyces cerevisiae Proteins / Q000737:chemistry
C078505:MRS1 protein, S cerevisiae; D024101:Mitochondrial Proteins; D016601:RNA-Binding Proteins; D012261:Ribonucleoproteins; D029701:Saccharomyces cerevisiae Proteins; D012313:RNA; D009713:Nucleotidyltransferases; C034038:mRNA maturase; D004722:Endoribonucleases