Halo Tag: Difference between revisions

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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 <ref>https://doi.org/10.1002/anie.202424955</ref>. 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.

==== ~~Function~~ ====

==== 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 ====

----

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*Protein Scaffold: An engineered HaloTag protein with the light-sensing sAsLOV2 domain.

*Ligand: Binds to a fluorogenic rhodamine dye (~~e.g.,~~ JF635-HTL).

*Ligand: Binds to a fluorogenic rhodamine dye (JF635-HTL).

:Light ON (450 nm): Light activates the sAsLOV2 domain using FMN cofactor, causing a conformational change (Jα helix unfolding).

: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.

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[[Image:Principle of HaloTag.jpg]]

~~==== Mechanism ====~~

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).

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 <StructureSection load='1stp' size='340' side='right' caption='Caption for this structure' scene=''>
+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 <ref>https://doi.org/10.1002/anie.202424955</ref>. 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.
-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 <ref>https://doi.org/10.1002/anie.202424955</ref>. 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.
-==== Function ====
+==== 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 ====
 ----
@@ Line 12: / Line 21: @@
 *Protein Scaffold: An engineered HaloTag protein with the light-sensing sAsLOV2 domain.
-*Ligand: Binds to a fluorogenic rhodamine dye (e.g., JF635-HTL).
+*Ligand: Binds to a fluorogenic rhodamine dye (JF635-HTL).
-:Light ON (450 nm): Light activates the sAsLOV2 domain using FMN cofactor, causing a conformational change (Jα helix unfolding).
+: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.
@@ Line 20: / Line 29: @@
 [[Image:Principle of HaloTag.jpg]]
-==== Mechanism ====
+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).