Visualization of the Orai1 homodimer and the functional coupling of Orai1-STIM1 by live-cell fluorescence lifetime imaging

Thiol dependent intramolecular locking of Orai1 channels

Store-operated Ca(2+) entry mediated by STIM1-gated Orai1 channels is essential to activate immune cells and its inhibition or gain-of-function can lead to immune dysfunction and other pathologies. Reactive oxygen species interacting with cysteine residues can alter protein function. Pretreatment of the Ca(2+) selective Orai1 with the oxidant H2O2 reduces ICRAC with C195, distant to the pore, being its major redox sensor. However, the mechanism of inhibition remained elusive. Here we combine experimental and theoretical approaches and show that oxidation of Orai1 leads to reduced subunit interaction, slows diffusion and that either oxidized C195 or its oxidomimetic mutation C195D located at the exit of transmembrane helix 3 virtually eliminates channel activation by intramolecular interaction with S239 of transmembrane helix 4, thereby locking the channel in a closed conformation. Our results demonstrate a novel mechanistic model for ROS-mediated inhibition of Orai1 and identify a candidate residue for pharmaceutical intervention.

Application of fluorescence resonance energy transfer in protein studies

Since the physical process of fluorescence resonance energy transfer (FRET) was elucidated more than six decades ago, this peculiar fluorescence phenomenon has turned into a powerful tool for biomedical research due to its compatibility in scale with biological molecules as well as rapid developments in novel fluorophores and optical detection techniques. A wide variety of FRET approaches have been devised, each with its own advantages and drawbacks. Especially in the last decade or so, we are witnessing a flourish of FRET applications in biological investigations, many of which exemplify clever experimental design and rigorous analysis. Here we review the current stage of FRET methods development with the main focus on its applications in protein studies in biological systems, by summarizing the basic components of FRET techniques, most established quantification methods, as well as potential pitfalls, illustrated by example applications.

Effect of Ca²⁺ on the steady-state and time-resolved emission properties of the genetically encoded fluorescent sensor CatchER

We previously designed a calcium sensor CatchER (a GFP-based Calcium sensor for detecting high concentrations in the high calcium concentration environment such as ER) with a capability for monitoring calcium ion responses in various types of cells. Calcium binding to CatchER induces the ratiometric changes in the absorption spectra, as well as an increase in fluorescence emission at 510 nm upon excitation at both 395 and 488 nm. Here, we have applied the combination of the steady-state and time-resolved optical methods and Hydrogen/Deuterium isotope exchange to understand the origin of such calcium-induced optical property changes of CatchER. We first demonstrated that calcium binding results in a 44% mean fluorescence lifetime increase of the indirectly excited anionic chromophore. Thus, CatchER is the first protein-based calcium indicator with the single fluorescent moiety to show the direct correlation between the lifetime and calcium binding. Calcium exhibits a strong inhibition on the excited-state proton transfer nonadiabatic geminate recombination in protic (vs deuteric) medium. Analysis of CatchER crystal structures and the MD simulations reveal the proton transfer mechanism in which the disrupted proton migration path in CatchER is rescued by calcium binding. Our finding provides important insights for a strategy to design calcium sensors and suggests that CatchER could be a useful probe for FLIM imaging of calcium in situ.