FLIM-FRET Protein-Protein Interaction Assay.

Fluorescence lifetime imaging performed under FRET conditions between two interacting molecules is a sensitive and robust way to quantify intermolecular interactions in cells. The fluorescence lifetime, an inherent property of the fluorophore, remains unaffected by factors such as concentration, laser intensity, and other photophysical artifacts. In the context of FLIM-FRET, the focus lies on measuring the fluorescence lifetime of the donor molecule, which diminishes upon interaction with a neighboring acceptor molecule. In this study, we present a step-by-step experimental protocol for applying FLIM-FRET to investigate protein-protein interactions involving various RAS isoforms and RAS effectors at the live cell's plasma membrane. By utilizing the FRET pair comprising enhanced green fluorescent protein (eGFP) and fluorescent mCherry, we demonstrate that the proximity and possible nanoclustering of eGFP-tagged KRAS4b G12D and mCherry-tagged KRAS4b WT led to a reduction in the donor eGFP's fluorescence lifetime. The donor lifetime of eGFP-tagged KRAS decreases even further when treated with a dimer-inducing small molecule, or in the presence of RAF proteins, suggesting a greater FRET efficiency, and thus less distance, between donor and acceptor.

Natural Product Graveoline Modulates Kirsten Rat Sarcoma Viral Oncogene Homologue (KRAS) Membrane Association: Insights from Advanced Spectroscopic Studies.

The KRAS gene plays a pivotal role in numerous cancers by encoding a GTPase that upon association with the plasma membrane activates the MAPK pathway, promoting cellular proliferation. In our study, we investigated small molecules that disrupt KRAS's membrane interaction, hypothesizing that such disruption could in turn inhibit mutant RAS signaling. Native mass spectrometry screening of KRAS-FMe identified compounds with a preference for interacting with the hypervariable region (HVR), and surface plasmon resonance (SPR) further refined our selection to graveoline as a compound exhibiting preferential HVR binding. Subsequent nuclear magnetic resonance (NMR) analysis showed that graveoline's interaction with KRAS depends on C-terminal O-methylation. Moreover, our findings revealed multiple interaction sites, suggesting weak engagement with the KRAS G domain. Using nanodiscs as a membrane mimetic, further characterization through NMR and Förster resonance energy transfer (FRET) studies demonstrated graveoline's ability to perturb KRAS membrane interaction in a biochemical setting. Our biophysical approach sheds light on the intricate molecular mechanisms underlying KRAS-ligand interactions, providing valuable insights into understanding the KRAS-associated pathophysiology. These findings contribute to the translational aspect of our study, offering potential avenues for further research targeting KRAS membrane association with the potential to lead to a new class of RAS therapeutics.

Virtual screening of ultra-large chemical libraries identifies cell-permeable small-molecule inhibitors of a "non-druggable" target, STAT3 N-terminal domain.

STAT3 N-terminal domain is a promising molecular target for cancer treatment and modulation of immune responses. However, STAT3 is localized in the cytoplasm, mitochondria, and nuclei, and thus, is inaccessible to therapeutic antibodies. Its N-terminal domain lacks deep pockets on the surface and represents a typical "non-druggable" protein. In order to successfully identify potent and selective inhibitors of the domain, we have used virtual screening of billion structure-sized virtual libraries of make-on-demand screening samples. The results suggest that the expansion of accessible chemical space by cutting-edge ultra-large virtual compound databases can lead to successful development of small molecule drugs for hard-to-target intracellular proteins.