Determination of Equilibrium Constant and Relative Brightness in FRET-FCS by Including the Third-Order Correlations

FRET-FCS: Advancing comprehensive insights into complex biological systems

Förster resonance energy transfer (FRET) is a short-range distance-dependent photophysical phenomenon that allows the measurement of intra- and intermolecular distances through fluorescence detection. FRET measurements are sensitive to the movements of fluorescently labeled molecules as they produce fluorescence fluctuations. Fluorescence correlation spectroscopy (FCS) analyzes these fluctuations at faster and broader timescales (from picoseconds to seconds) compared with other techniques, unraveling the thermodynamic and kinetic properties of the system under study. Therefore, the combination of FRET and FCS (FRET-FCS) facilitates the analysis of molecular dynamics. Since its introduction, FRET-FCS has evolved into studying more sophisticated systems, requiring improvements in data acquisition and analysis. In this review, we discuss applications in the field of FRET-FCS that propose novel alternatives to overcome the inherent limitations of experimental setups. This work aims to promote using and enhancing FRET-FCS techniques to develop a comprehensive understanding of biological systems.

Affinity of Skp to OmpC revealed by single-molecule detection

Outer membrane proteins (OMPs) are essential to gram-negative bacteria, and molecular chaperones prevent the OMPs from aggregation in the periplasm during the OMPs biogenesis. Skp is one of the molecular chaperones for this purpose. Here, we combined single-molecule fluorescence resonance energy transfer and fluorescence correlation spectroscopy to study the affinity and stoichiometric ratio of Skp in its binding with OmpC at the single-molecule level. The half concentration of the Skp self-trimerization (C_1/2) was measured to be (2.5 ± 0.7) × 10² nM. Under an Skp concentration far below the C_1/2, OmpC could recruit Skp monomers to form OmpC·Skp₃. The affinity to form the OmpC·Skp₃ complex was determined to be (5.5 ± 0.4) × 10² pM with a Hill coefficient of 1.6 ± 0.2. Under the micromolar concentrations of Skp, the formation of OmpC·(Skp₃)₂ was confirmed, and the dissociation constant of OmpC·(Skp₃)₂ was determined to be 1.2 ± 0.4 μM. The precise information will help us to quantitatively depict the role of Skp in the biogenesis of OMPs.

Single-molecule study on conformational dynamics of M.HhaI

We found that apo DNA methyltransferase M.HhaI under the physiological salt concentration does not possess the structure characterized by X-ray crystallography; instead, it interchanges between prefolded and unfolded states. Only after binding to the substrate, it transforms into a crystal-structure-like state. Flipping rates of its catalytic loop were directly measured.

Huge conformational rearrangements in M.HhaI were observed by a single-molecule study.