Dataset Information

Single-Molecule Analysis of Cytochrome c Folding by Monitoring the Lifetime of an Attached Fluorescent Probe.

ABSTRACT: Conformational dynamics of proteins are important for function. However, obtaining information about specific conformations is difficult for samples displaying heterogeneity. Here, time-resolved fluorescence resonance energy transfer is used to characterize the folding of single cytochrome c molecules. In particular, measurements of the fluorescence lifetimes of individual cytochrome c molecules labeled with a single dye that is quenched by energy transfer to the heme were used to monitor conformational transitions of the protein under partially denaturing conditions. These studies indicate significantly more conformational heterogeneity than has been described previously. Importantly, the use of a purified singly-labeled sample made a direct comparison to ensemble data possible. The distribution of lifetimes of single-proteins was compared to the distribution of lifetimes determined from analysis of ensemble lifetime fluorescence data. The results show broad agreement between single-molecule and ensemble data, with a similar range of lifetimes. However, the single-molecule data reveal greater conformational heterogeneity.

SUBMITTER: Lee AJ

PROVIDER: S-EPMC3791595 | biostudies-literature | 2013 Aug

REPOSITORIES: biostudies-literature

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Single-Molecule Analysis of Cytochrome c Folding by Monitoring the Lifetime of an Attached Fluorescent Probe.

Lee Andrea J AJ Asher Wesley B WB Stern Harry A HA Bren Kara L KL Krauss Todd D TD

The journal of physical chemistry letters 20130801 16

Conformational dynamics of proteins are important for function. However, obtaining information about specific conformations is difficult for samples displaying heterogeneity. Here, time-resolved fluorescence resonance energy transfer is used to characterize the folding of single cytochrome c molecules. In particular, measurements of the fluorescence lifetimes of individual cytochrome c molecules labeled with a single dye that is quenched by energy transfer to the heme were used to ...[more]

PMID: 24116268

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Project description:We describe the theory, experiment, and analysis of three-color Förster resonance energy transfer (FRET) spectroscopy for probing conformational dynamics of a fast-folding protein, α3D. In three-color FRET, site-specific labeling of fluorophores is required to avoid ambiguity resulting from various species with different combinations of labeling positions. To this end, we first attached two dyes to a cysteine residue and an unnatural amino acid and then appended a cysteine residue to the C-terminus of the protein by the sortase-mediated ligation for attaching the third dye. To determine all three FRET efficiencies, we used alternating excitation of the donor and acceptor 1 with two picosecond-pulsed lasers. Since the folded and unfolded states are not distinguishable in binned fluorescence trajectories due to fast-folding on a millisecond time scale, we used a maximum likelihood method that analyzes photon trajectories without binning the data. The extracted kinetic parameters agree very well with the previously measured parameters for the same protein with two-color FRET, suggesting that the addition of the third fluorophore does not affect the folding dynamics of the protein. From the extracted fractions of acceptor photon counts, the FRET efficiencies for all three dye pairs were calculated after various corrections. They were compared with the FRET efficiencies obtained from the global analysis of two-color segments collected in the same experiment. The FRET efficiencies of the folded state from the three-color segments agree with those from the two-color segments, whereas the three-color and two-color FRET efficiencies of the unfolded state are different. This happens because fluctuations of all three interdye distances contribute to the FRET efficiency measured in three-color FRET. We show that this difference can be accounted for by using the Gaussian chain model for the unfolded state with the parameters obtained from the analysis of two-color segments. This result shows that three-color FRET provides additional information on the flexibility of molecules that cannot be obtained from a combination of two-color FRET experiments with three dye pairs. Using the delay times of photons from the laser pulse, fluorescence lifetimes were determined using the maximum likelihood analysis. The correlation between FRET efficiencies and lifetimes of the donor, acceptor 1, and acceptor 2 was visualized in two-dimensional FRET efficiency-lifetime histograms. These histograms can be used to demonstrate the presence of conformational dynamics in a protein.

Dataset Information

Single-Molecule Analysis of Cytochrome c Folding by Monitoring the Lifetime of an Attached Fluorescent Probe.

Publications

Single-Molecule Analysis of Cytochrome <i>c</i> Folding by Monitoring the Lifetime of an Attached Fluorescent Probe.

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