The autocatalytic maturation of the chromophore in green fluorescent protein (GFP) was thought to require the precise positioning of the side chains surrounding it in the core of the protein, many of which are strongly conserved among homologous fluorescent proteins. In this study, we screened for green fluorescence in an exhaustive set of point mutations of seven residues that make up the chromophore microenvironment, excluding R96 and E222 because mutations at these positions have been previously characterized. Contrary to expectations, nearly all amino acids were tolerated at all seven positions. Only four point mutations knocked out fluorescence entirely. However, chromophore maturation was found to be slower and/or fluorescence reduced in several cases. Selected combinations of mutations showed nonadditive effects, including cooperativity and rescue. The results provide guidelines for the computational engineering of GFPs.

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Split fluorescent proteins have been engineered for various purposes, in each case signaling their spontaneous reconstitution by fluorescence. By combining split protein reconstitution and computational protein design, we have constructed a circularly permuted and truncated variant of green fluorescent protein (GFP) in which the seventh beta strand has been left out and the sites around it computationally designed to accommodate a peptide from influenza hemagglutinin. We call this a ''leave-one-out'' GFP biosensor (LOO-GFP). A LOO-GFP was designed using DEEdesign, selected by plate screening a bacterial library in the presence of the influenza peptide target, and was found to have seven point mutations. But binding was weak (9mM) and was at the expense of stability. The weakened, partially folded protein aggregated in the absence of its target. Furthermore, the aggregated biosensor fluoresced more than its monomeric peptide-bound form. In this work, the LOO-GFP was rationally redesigned to fold more robustly and bind the target tighter. Modeling of the GFP folding pathway suggested that one of the seven mutations, F83W, interfered with the closing of the beta barrel. Mutating this residue back to a F indeed, along with several other rationally justified changes followed by re-screening, produced several biosensor sequences with slower unfolding rates, a positive binding signal, and higher chromophore maturation efficiency. We also observed a blue-shift in the excitation spectrum, and lower background fluorescence in the unbound state. Kd was unchanged. In parallel experiments, LOO-GFP biosensors were genetically fused to fibers formed by the Drosophila protein ultrabithorax (Ubx), and were found to be absent any background fluorescence in the unbound state, but recovered fluorescence when exposed to the target peptide. Implications for the design of biosensing materials are discussed. Glucokinase (GCK) is the rate-limiting enzyme for glucose metabolism in glucose-sensing tissues, including hypothalamic nuclei. Post-translational regulation of GCK by receptor-mediated signaling pathways has been identified in the liver and the pancreas, and is critical for the maintenance of glucose homeostasis; yet, it is unclear if a similar regulation of GCK occurs in hypothalamic neurons. Using the hypothalamically-derived, glucosensing GT1-7 neuronal cell line, the existence of a receptor-mediated signaling cascade culminating in the S-nitrosylation and activation of GCK is demonstrated. GCK activity is assessed by either measuring NAD(P)H autofluorescence while raising extracellular glucose, or through expression of a FRET GCK biosensor. Treatment of GT1-7 cells with isoproterenol, a G s GPCR agonist, augments a glucose-dependent rise in NAD(P)H autofluorescence, and activates the GCK biosensor. Applying the nitric oxide synthase inhibitor L-NAME impedes the metabolic effect of isoproterenol. Additionally, incorporation of an S-nitrosylation-blocking V367M mutation into the biosensor prevents GCK activation by isoproterenol....