Hierarchical Folding Research Articles

Characterization of the Tetrahymena ribozyme folding pathway using the kinetic footprinting reagent peroxynitrous acid.

Large RNAs fold into complex structures which determine their biological activities. A full understanding of both RNA structure and dynamics will include the description of the pathways by which these structures are formed. Kinetic footprinting [Sclavi, B., et al. (1997) J. Mol. Biol. 266, 144-159] has been shown to be a powerful method for the study of dynamic processes involving RNA. Here we describe the use of a readily available reagent, peroxynitrous acid, as a kinetic footprinting tool for the study of RNA folding. Hydroxyl radicals generated from this reagent were used to footprint the Tetrahymena ribozyme during its magnesium-dependent folding-in agreement with synchroton X-ray footprinting [Sclavi, B., et al. (1998) Science 279, 1940-1943] and oligonucleotide/hybridization cleavage experiments [Zarrinkar, P. P., and Williamson, J. R. (1994) Science 265, 918-924], this work suggests an ordered, hierarchical folding pathway for the ribozyme. Several slow steps in the folding pathway were observed in the peroxynitrous acid footprinting, but none of these corresponded to the rate-determining step of folding. This suggests that the formation of the global, protected structure is followed by one or more slow local rearrangements to yield the final active structure. These studies illustrate the utility of peroxynitrous acid as a reagent for the elucidation of RNA folding pathways and the study of RNA dynamics.

An Interactive Framework for RNA Secondary Structure Prediction with a Dynamical Treatment of Constraints

A novel approach aiding in the prediction of RNA secondary structures is presented. Although phylogenetic methods are the most successful at deriving RNA secondary structures, the are not applicable when the number of sequences or the sequence variability is too low. Methods based on energy minimization are therefore of great interest. However, some of the suboptimal RNA secondary structures computed with classic methods are unsaturated structures, i.e. some structures are included into others. Thus, the incorporation of constraints during the process of folding is not possible, while the incorporation of constraints before the process of folding often introduces a bias into the energy function. This paper describes a new procedure which allows for the incorporation of constraints before and during the process of RNA folding. SAPSSARN is an interactive program which offers a framework, both to specify a secondary structure through a set of folding constraints and to compute all the supoptimal saturated RNA secondary structures which satisfy all the folding constraints. At the start, it relies on the computation of the probabilities of pairing of each base with all others according to McCaskill's algorithm. The constraint satisfaction formulation of the problem deals dynamically with a chosen set of folding constraints and, finally, a search algorithm computes all the suboptimal saturated secondary structures which satisfy those folding constraints. Within such a framework, it is possible to test new ideas about RNA folding and secondary structures, including pseudoknots, can be computed. The program is illustrated with RNA sequences on which we obtained results in agreement with known structures by using a protocol which mimics the hierarchical folding of RNA molecules.

Folding units in globular proteins.

We present a method to identify all compact, contiguous-chain, structural units in a globular protein from x-ray coordinates. These units are then used to describe a complete set of hierarchic folding pathways for the molecule. Our analysis shows that the larger units are combinations of smaller units, giving rise to a structural hierarchy ranging from the whole protein monomer through supersecondary structures down to individual helices and strands. It turns out that there is more than one way to assemble the protein by self-association of its compact units. However, the number of possible pathways is small--small enough to be exhaustively explored by a computer program. The hierarchic organization of compact units in protein molecules is consistent with a model for folding by hierarchic condensation. In this model, neighboring hydrophobic chain sites interact to form folding clusters, with further stepwise cluster association giving rise to a population of folding intermediates.