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Mechanical unfolding of RNA: from hairpins to structures with internal multiloops.

ABSTRACT: Mechanical unfolding of RNA structures, ranging from hairpins to ribozymes, using laser optical tweezer experiments have begun to reveal the features of the energy landscape that cannot be easily explored using conventional experiments. Upon application of constant force (f), RNA hairpins undergo cooperative transitions from folded to unfolded states whereas subdomains of ribozymes unravel one at a time. Here, we use a self-organized polymer model and Brownian dynamics simulations to probe mechanical unfolding at constant force and constant-loading rate of four RNA structures of varying complexity. For simple hairpins, such as P5GA, application of constant force or constant loading rate results in bistable cooperative transitions between folded and unfolded states without populating any intermediates. The transition state location (DeltaxFTS) changes dramatically as the loading rate is varied. At loading rates comparable to those used in laser optical tweezer experiments, the hairpin is plastic, with DeltaxFTS being midway between folded and unfolded states; whereas at high loading rates, DeltaxFTS moves close to the folded state, i.e., RNA is brittle. For the 29-nucleotide TAR RNA with the three-nucleotide bulge, unfolding occurs in a nearly two-state manner with an occasional pause in a high free energy metastable state. Forced unfolding of the 55 nucleotides of the Hepatitis IRES domain IIa, which has a distorted L-shaped structure, results in well-populated stable intermediates. The most stable force-stabilized intermediate represents straightening of the L-shaped structure. For these structures, the unfolding pathways can be predicted using the contact map of the native structures. Unfolding of a RNA motif with internal multiloop, namely, the 109-nucleotide prohead RNA that is part of the 29 DNA packaging motor, at constant value of rf occurs with three distinct rips that represent unraveling of the paired helices. The rips represent kinetic barriers to unfolding. Our work shows 1), the response of RNA to force is largely determined by the native structure; and 2), only by probing mechanical unfolding over a wide range of forces can the underlying energy landscape be fully explored.

SUBMITTER: Hyeon C

PROVIDER: S-EPMC1779982 | biostudies-literature | 2007 Feb

REPOSITORIES: biostudies-literature

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Mechanical unfolding of RNA: from hairpins to structures with internal multiloops.

Hyeon Changbong C Thirumalai D D

Biophysical journal 20061006 3

Mechanical unfolding of RNA structures, ranging from hairpins to ribozymes, using laser optical tweezer experiments have begun to reveal the features of the energy landscape that cannot be easily explored using conventional experiments. Upon application of constant force (f), RNA hairpins undergo cooperative transitions from folded to unfolded states whereas subdomains of ribozymes unravel one at a time. Here, we use a self-organized polymer model and Brownian dynamics simulations to probe mecha ...[more]

PMID: 17028142

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Project description:We present an approach for globally monitoring RNA structure in native conditions in vivo with single nucleotide precision. This method is based on in vivo modification with dimethyl sulfate (DMS), which reacts with unpaired adenine and cytosine residues9, followed by deep sequencing to monitor modifications. Our data from yeast and mammalian cells are in excellent agreement with known mRNA structures and with the high-resolution crystal structure of the Saccharomyces cerevisiae ribosome10. Comparison between in vivo and in vitro data reveals that in rapidly dividing cells there are vastly fewer structured mRNA regions in vivo than in vitro. Even thermostable RNA structures are often denatured in cells, highlighting the importance of cellular processes in regulating RNA structure. Indeed, analysis of mRNA structure under ATP-depleted conditions in yeast reveals that energy-dependent processes strongly contribute to the predominantly unfolded state of mRNAs inside cells. Our studies broadly enable the functional analysis of physiological RNA structures and reveal that, in contrast to the Anfinsen view of protein folding, thermodynamics play an incomplete role in determining mRNA structure in vivo. We use Dimethyl Sulfate to probe the structure of rRNA and mRNA in yeast in vivo, in vitro, and at different temperatures in vitro. We obtain a great agreement between in vivo data and known mRNA structures as well as the ribosome crystal structure. We find that in contrast to ribosomal rna, mRNAs are less structured in vivo than in vitro, and the structures present in vivo can only partially be explained by thermodynamic stability. In addition, we identify new regulatory structures present in vivo. Examination of RNA structure in yeast under different conditions - in vivo and in vitro at five different temperatures (30,45,60,75,95) We adapt our DMS-seq assay for use in mammalian cells and probe RNA structure genome-wide in K562 cells. We probe the RNA structure of primary fibroblast using DMS on a genome-wide scale to confirm the presence of more structures in vitro. In addition we probe the RNA structure in yeast upon ATP depleted conditions to investigate whether active (ATP-dependent) processed are modulating RNA structure in vivo.

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Mechanical unfolding of RNA: from hairpins to structures with internal multiloops.

Publications

Mechanical unfolding of RNA: from hairpins to structures with internal multiloops.

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