Halo Tag
Introduction
The [Photoswitchable Halo Tag] (psHaloTag) is a unique chemigenetic system designed to provide reversible, light-controlled fluorescence tunable through both genetic and synthetic modifications, with promising applications for dynamic imaging in biological microscopy [1]. psHaloTag addresses a major gap in the availability of reliable, reversible systems suitable for live-cell imaging. Many current systems are limited in tracking long-term, cyclical processes or in refreshing the pool of observable molecules. psHaloTag combines the genetically encoded HaloTag protein with the light-sensing sAsLOV2 domain. When illuminated with 450 nm light, psHaloTag undergoes a reversible change that activates a bound rhodamine dye ligand, resulting in a significant increase in fluorescence. This strong, multiple-cycle reporter overcomes the limitations of irreversible systems. It is an important tool for achieving precise spatiotemporal control in demanding applications such as live-cell Super-Resolution Microscopy (SMLM), where many established photosensitive probes cannot be reactivated.
Features
- Photoswitching | Reversible ON/OFF fluorescence control using 450 nm light.
- Chemigenetic | Genetically encoded protein plus high-performance synthetic dye.
- Live-Cell Ready | Robust, visible-light activation across various targets.
Mechanism
The psHaloTag works as an allosteric photoswitch.
- Protein Scaffold: An engineered HaloTag protein with the light-sensing sAsLOV2 domain.
- Ligand: Binds to a fluorogenic rhodamine dye (JF635-HTL).
- Light ON (450 nm): Light activates the sAsLOV2 domain using Flavin Mononucleotide (FMN)
cofactor, causing a conformational change (Jα helix unfolding).
- Fluorescence ON: This change forces the bound dye into its fluorescent state.
- Light OFF (Dark): The protein structure relaxes back to its original shape, shifting
the dye back to its non-fluorescent state.
The psHaloTag variants show a strong, reversible, deep-red fluorescence turn-on when activated by light. This provides a well-controlled tool for improved imaging.
Structural highlights
The protein is made by inserting the light-sensing sAsLOV2 domain into the HaloTag protein. An extended helix connects the domains. This connection makes sure the light-induced structural change from the LOV domain is effectively transmitted to the HaloTag region, where the rhodamine dye is bound. Performance-boosting mutations are found near the headgroup of the rhodamine dye. These mutations likely stabilize the interaction to enhance the fluorescence ON/OFF ratio.
Relevance
Although irreversibly photoactivatable fluorophores are well established, reversible reporters that can be reactivated multiple times remain scarce, and only a few have been applied in living cells using generalizable protein labelling methods. This system improves techniques like SMLM by allowing precise control of single-molecule emitter density over time by achieving sub-diffraction resolution in living cells. Also, it exploits high-quality synthetic dyes for bright colours and stability in light whose performance does not rely on oxygen and is less affected by pH changes compared to traditional fluorescent proteins (FPs).
<Structure load='9HKF' size='350' frame='true' align='right' caption='Crystal structure of psHaloTag labeled with JF635-HTL' </StructureSection>
