A fluorogenic chemically induced dimerization technology for controlling, imaging and sensing protein proximity

Subcellular imaging of lipids and sugars using genetically encoded proximity sensors

Live cell imaging of lipids and other metabolites is a long-standing challenge in cell biology. Bioorthogonal labeling tools allow for the conjugation of fluorophores to several phospholipid classes, but cannot discern their trafficking between adjacent organelles or asymmetry across individual membrane leaflets. Here we present fluorogen-activating coincidence sensing (FACES), a chemogenetic tool capable of quantitatively imaging subcellular lipid pools and reporting their transbilayer orientation in living cells. FACES combines bioorthogonal chemistry with genetically encoded fluorogen-activating proteins (FAPs) for reversible proximity sensing of conjugated molecules. We first validate this approach for quantifying discrete phosphatidylcholine pools in the ER and mitochondria that are trafficked by lipid transfer proteins. We then show that transmembrane domain-containing FAPs can be used to reveal the membrane asymmetry of multiple lipid classes that are generated in the trans-Golgi network. Lastly, we demonstrate that FACES is a generalizable tool for subcellular bioorthogonal imaging by measuring changes in mitochondrial N -acetylhexosamine levels. These results demonstrate the use of fluorogenic tags for spatially-defined molecular imaging.

A multi-colour fluorogenic tag and its application in Candida albicans

Fluorescent proteins (FPs) have always been a crucial part of molecular research in life sciences, including the research into the human fungal pathogen Candida albicans, but have obvious shortcomings such as their relatively large size and long maturation time. However, the next generation of FPs overcome these issues and rely on the binding of a fluorogen for the protein to become fluorescently active. This generation of FPs includes the improved version of Fluorescence activating and Absorption Shifting Tag (iFAST). The binding between the fluorogen and the iFAST protein is reversible, thus resulting in reversible fluorescence. The fluorogens of iFAST are analogues of 4-hydroxylbenzylidene-rhodanine (HBR). These HBR analogues differ in spectral properties depending on functional group substitutions, which gives the iFAST system flexibility in terms of absorbance and emission maxima. In this work we describe and illustrate the application of iFAST as a protein tag and its reversible multi-colour characteristics in C. albicans.

Engineering of Tunable Allosteric-like Fluorogenic Protein Sensors

Optical protein sensors that enable detection of relevant biomolecules of interest play central roles in biological research. Coupling fluorescent reporters with protein sensing units has enabled the development of a wide range of biosensors that recognize analytes with high selectivity. In these sensors, analyte recognition induces a conformational change in the protein sensing unit that can modulate the optical signal of the fluorescent reporter. Here, we explore various designs for the creation of tunable allosteric-like fluorogenic protein sensors through incorporation of sensing protein units within the chemogenetic fluorescence-activating and absorption-shifting tag (FAST) that selectively binds and stabilizes the fluorescent state of 4-hydroxybenzylidene rhodanine (HBR) analogs. Conformational coupling allowed us to design analyte-responsive optical protein sensors through allosteric-like modulation of fluorogen binding.

Microbiome imaging goes à la carte: Incorporating click chemistry into the fluorescence-activating and absorption-shifting tag (FAST) imaging platform

The human microbiome is predominantly composed of facultative and obligate anaerobic bacteria that live in hypoxic/anoxic polymicrobial biofilm communities. Given the oxidative sensitivity of large fractions of the human microbiota, green fluorescent protein (GFP) and related genetically-encoded fluorophores only offer limited utility for live cell imaging due the oxygen requirement for chromophore maturation. Consequently, new fluorescent imaging modalities are needed to study polymicrobial interactions and microbiome-host interactions within anaerobic environments. The fluorescence-activating and absorption shifting tag (FAST) is a rapidly developing genetically-encoded fluorescent imaging technology that exhibits tremendous potential to address this need. In the FAST system, fluorescence only occurs when the FAST protein is complexed with one of a suite of cognate small molecule fluorogens. To expand the utility of FAST imaging, we sought to develop a modular platform (Click-FAST) to democratize fluorogen engineering for personalized use cases. Using Click-FAST, investigators can quickly and affordably sample a vast chemical space of compounds, potentially imparting a broad range of desired functionalities to the parental fluorogen. In this work, we demonstrate the utility of the Click-FAST platform using a novel fluorogen, PLBlaze-alkyne, which incorporates the widely available small molecule ethylvanillin as the hydroxybenzylidine head group. Different azido reagents were clicked onto PLBlaze-alkyne and shown to impart useful characteristics to the fluorogen, such as selective bacterial labeling in mixed populations as well as fluorescent signal enhancement. Conjugation of an 80 Å PEG molecule to PLBlaze-alkyne illustrates the broad size range of functional fluorogen chimeras that can be employed. This PEGylated fluorogen also functions as an exquisitely selective membrane permeability marker capable of outperforming propidium iodide as a fluorescent marker of cell viability.

Fluorogenic chemically induced dimerization

A new chemically induced dimerization (CID) pair exhibits fluorescence upon dimerization for the first time. Moreover, the CID pair is small in size and offers easily reversible dimerization that can be repeated multiple times.