Biarsenical ligands bind to endogenous G-protein α-subunits and enable allosteric sensing of nucleotide binding

BACKGROUND

Heterotrimeric G-proteins relay extracellular signals to intracellular effector proteins. Multiple methods have been developed to monitor their activity; including labeled nucleotides and biosensors based on genetically engineered G-proteins. Here we describe a method for monitoring unlabeled nucleotide binding to endogenous G-proteins α-subunits in a homogeneous assay based on the interaction of 4',5'-bis(1,2,3-dithioarsolan-2-yl)-2',7'-difluorofluorescein (F2FlAsH) with G-protein α-subunits.

RESULTS

The biarsenic fluorescent ligand F2FlAsH binds to various wild-type G-protein α-subunits (αi1, αi2, αi3, αslong, αsshort, αolf, αq, α13) via high affinity As-cysteine interactions. This allosteric label enables real time monitoring of the nucleotide bound states of α-subunits via changes in fluorescence anisotropy and intensity of their F2FlAsH-complexes. We have found that different α-subunits displayed different signal amplitudes when interacting with F2FlAsH, being more sensitive to nucleotide binding to αi, αs, αolf and αq than to α13. Addition of nucleotides to F2FlAsH-labeled α-subunits caused concentration-dependent effects on their fluorescence anisotropy. pEC50 values of studied nucleotides depended on the subtype of the α-subunit and were from 5.7 to 8.2 for GTPγS, from 5.4 to 8.1 for GppNHp and from 4.8 to 8.2 for GDP and lastly up to 5.9 for GMP. While GDP and GMP increased the fluorescence anisotropy of F2FlAsH complexes with αi-subunits, they had the opposite effect on the other αβγM complexes studied.

CONCLUSIONS

Biarsenical ligands interact allosterically with endogenous G-protein α-subunits in a nucleotide-sensitive manner, so the presence or absence of guanine nucleotides has an effect on the fluorescence anisotropy, intensity and lifetime of F2FlAsH-G-protein complexes.

Development of Cubosomes as a Cell-Free Biosensing Platform

The parallel between the lipidic microenvironments of the inverse bicontinuous cubic phase and the biological membrane distinguishes cubic phases as an attractive option for development of cell-free biosensors containing protein or glycolipid receptors. Herein we describe a novel strategy toward the creation of a biosensing platform derived from the surface attachment of a colloidally stable inverse cubic structure (cubosomes). We report the preparation of cubosomes composed of the amphiphile phytantriol, the membrane glycolipid receptor monosialoganglioside-GM1 and the biotin-functionalized amphiphile 1,2-distearoyl-sn-glycero-3-phosphoethanolamine-N-[biotinyl(polyethyleneglycol)-2000] (bDSPE). The tethering of cubosomes to the various surfaces was mediated through bDSPE binding to streptavidin- and avidin-modified surfaces. Allylamine plasma polymer surface modification enhanced the surface immobilization of avidin, which increased the density of bound cubosomes. The resultant polymer–protein–cubosome complex was imaged by cryo-transmission electron microscopy analysis and the cubosome structure was impressively preserved within the complex. Cholera toxin binding to cubosomes containing GM1 was used to assess the performance of the cubosomes, subsequent to surface attachment, via a modified enzyme-linked immunosorbent assay. Specific immobilization of complex protein–receptor–cubosome systems paves the way for development of a structurally complex, heterogeneous platform for sensing applications.

Imaging-based approaches to understanding g protein-coupled receptor signalling complexes

In the last 10 years, imaging assays based on resonance energy transfer (RET) and protein fragment complementation have made it possible to study interactions between components of G protein-coupled receptor (GPCR) signalling complexes in living cells under physiological conditions. Here, we consider the history of such approaches, the current tools available and how they have changed our understanding of GPCR signalling. We also discuss some theoretical and methodological issues important when combining the different types of assay.