Abstract
Visualizing and quantifying molecular motion and interactions inside living cells provides crucial insight into the mechanisms underlying cell function. This has been achieved by super-resolution localization microscopy and single-molecule tracking in conjunction with photoactivatable fluorescent proteins (PA-FPs). An alternative labelling approach relies on genetically-encoded protein tags with cell-permeable fluorescent ligands which are brighter and less prone to photobleaching than fluorescent proteins but require a laborious labelling process. Either labelling method is associated with significant advantages and disadvantages that should be taken into consideration depending on the microscopy experiment planned. Here, we describe an optimised procedure for labelling Halo-tagged proteins in live Escherichia coli cells. We provide a side-by-side comparison of Halo tag with different fluorescent ligands against the popular photoactivatable fluorescent protein PAmCherry. Using test proteins with different intracellular dynamics, we evaluated fluorescence intensity, background, photostability, and results from single-molecule localization and tracking experiments. Capitalising on the brightness and extended spectral range of fluorescent Halo ligands, we also demonstrate high-speed and dual-colour single-molecule tracking.
Highlights
After optimising a protocol for Halo tag labelling, we tested labelling specificity and background, quantified the brightness and photostability of the labels, and performed tracking experiments on several different fusion proteins
The combination of super-resolution localization microscopy with single-particle tracking has proven very powerful as it allows direct visualization of the activity of thousands
Super-resolution localization microscopy is based on the sequential detection and localization of individual fluorescent
Summary
After optimising a protocol for Halo tag labelling, we tested labelling specificity and background, quantified the brightness and photostability of the labels, and performed tracking experiments on several different fusion proteins. We optimized a procedure for labelling Halo tag in live E. coli with the commercially available dye TMR (figure 1). To assess the level of non-specific labelling, we measured the localization frequency of TMR inside cells that do not express Halo tag.
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