Aberrant activation of the epidermal growth factor receptor (EGFR) is critical to the biology of many common cancers. The molecular events that define how EGFR transmits an extracellular ligand binding event through the membrane are not understood. Here we use a chemical tool, bipartite tetracysteine display, to report on ligand-specific conformational changes that link ligand binding and kinase activation for full-length EGFR on the mammalian cell surface. We discover that EGF binding is communicated to the cytosol through formation of an antiparallel coiled coil within the intracellular juxtamembrane (JM) domain. This interaction is functionally coupled to receptor activation by EGF. In contrast, TGFα binding is communicated to the cytosol through formation of a discrete, alternative helical interface. These findings suggest that the JM region can differentially decode extracellular signals and transmit them to the cell interior, and provide new insight into how EGFR communicates ligand-specific information across the membrane.

The green fluorescent protein (GFP) gene, gfp, of the jellyfish Aequorea victoria is being used as a reporter system for gene expression and as a marker for tracking prokaryotes and eukaryotes. Cells that have been genetically altered with the gfp gene produce a protein that fluoresces when it is excited by UV light. This unique phenotype allows gfp-tagged cells to be specifically monitored by nondestructive means. In this study we determined whether a gfp-tagged strain of Pseudomonas fluorescens continued to fluoresce under conditions under which the cells were starved, viable but nonculturable (VBNC), or dead. Epifluorescent microscopy, flow cytometry, and spectrofluorometry were used to measure fluorescence intensity in starved, VBNC, and dead or dying cells. Results obtained by using flow cytometry indicated that microcosms containing VBNC cells, which were obtained by incubation under stress conditions (starvation at 37.5°C), fluoresced at an intensity that was at least 80% of the intensity of nonstressed cultures. Similarly, microcosms containing starved cells incubated at 5 and 30°C had fluorescence intensities that were 90 to 110% of the intensity of nonstressed cells. VBNC cells remained fluorescent during the entire 6-month incubation period. In addition, cells starved at 5 or 30°C remained fluorescent for at least 11 months. Treatment of the cells with UV light or incubation at 39 or 50°C resulted in a loss of GFP from the cells. There was a strong correlation between cell death and leakage of GFP from the cells, although the extent of leakage varied depending on the treatment. Most dead cells were not GFP fluorescent, but a small proportion of the dead cells retained some GFP at a lower concentration than the concentration in live cells. Our results suggest that gfp-tagged cells remain fluorescent following starvation and entry into the VBNC state but that fluorescence is lost when the cells die, presumably because membrane integrity is lost.

Summary In recent years, scientists have expanded their focus from cataloging genes to characterizing the multiple states of their translated products. One anticipated result is a dynamic map of the protein association networks and activities that occur within the cellular environment. While in vitro-derived network maps can illustrate which of a multitude of possible protein-protein associations could exist, they supply a falsely static picture lacking the subtleties of subcellular location (where) or cellular state (when). Generating protein association network maps that are informed by both subcellular location and cell state requires novel approaches that accurately characterize the state of protein associations in living cells and provide precise spatiotemporal resolution. In this review, we highlight recent advances in visualizing protein associations and networks under increasingly native conditions. These advances include second generation protein complementation assays (PCAs), chemical and photo-crosslinking techniques, and proximity-induced ligation approaches. The advances described focus on background reduction, signal optimization, rapid and reversible reporter assembly, decreased cytotoxicity, and minimal functional perturbation. Key breakthroughs have addressed many challenges and should expand the repertoire of tools useful for generating maps of protein interactions resolved in both time and space.

Mutations in the EGFR kinase domain are implicated in non-small cell lung cancer. Of particular interest is the drug-resistant double mutant (L858R/T790M, DM EGFR), which is not inhibited selectively by any approved kinase inhibitor. Here we apply bipartite tetracysteine display to demonstrate that DM and WT EGFR differ in structure outside the kinase domain. The structural difference is located within the cytoplasmic juxtamembrane segment (JM) that links the kinase domain with the extracellular and transmembrane regions and is essential for EGFR activation. We show further that third-generation DM EGFR-selective TKIs alter JM structure via allostery to restore the conformation found when WT EGFR is activated by the growth factors EGF and HB-EGF. This work suggests that the oncogenic activity of DM EGFR may extend beyond kinase activity per se to include kinase-independent activities. As JM structure may provide a biomarker for these kinase-independent functions, these insights could guide the development of allosteric, DM-selective inhibitors.

Interception of the Pd-catalyzed decarboxylative allylation of allyl diphenylglycinate imines with appropriately functionalized Michael acceptors, followed by Heck cyclization, allows for the efficient construction of relatively complex organoamine frameworks in one reaction vessel. The initial intercepted decarboxylative allylation is remarkably insensitive toward solvent and catalyst, typically proceeding under ambient conditions.