Microbial transglutaminase (mTG) shows broad substrate specificity that is amenable to in vitro bio-conjugation applications. Herein, test proteins were genetically fused with peptide tags, followed by mTG-mediated propargylation of their reactive Gln residues. The propargylated proteins were subjected to copper-assisted azide-alkyne cycloaddition to demonstrate either fluorescent labelling or immobilization.
Red fluorescent proteins (RFPs) derived from organisms in the class Anthozoa have found widespread application as imaging tools in biological research. For most imaging experiments, RFPs that mature quickly to the red chromophore and produce little or no green chromophore are most useful. In this study, we used rational design to convert a yellow fluorescent mPlum mutant to a red-emitting RFP without reverting any of the mutations causing the maturation deficiency and without altering the red chromophore’s covalent structure. We also created an optimized mPlum mutant (mPlum-E16P) that matures almost exclusively to the red chromophore. Analysis of the structure/function relationships in these proteins revealed two structural characteristics that are important for efficient red chromophore maturation in DsRed-derived RFPs. The first is the presence of a lysine residue at position 70 that is able to interact directly with the chromophore. The second is an absence of non-bonding interactions limiting the conformational flexibility at the peptide backbone that is oxidized during red chromophore formation. Satisfying or improving these structural features in other maturation-deficient RFPs may result in RFPs with faster and more complete maturation to the red chromophore.
Transglutaminases (TGases) are enzymes that catalyse protein cross-linking through a transamidation reaction between the side chain of a glutamine residue on one protein and the side chain of a lysine residue on another. Generally, TGases show low substrate specificity with respect to their amine substrate, such that a wide variety of primary amines can participate in the modification of specific glutamine residue. Although a number of different TGases have been used to mediate these bioconjugation reactions, the TGase from Bacillus subtilis (bTG) may be particularly suited to this application. It is smaller than most TGases, can be expressed in a soluble active form, and lacks the calcium dependence of its mammalian counterparts. However, little is known regarding this enzyme and its glutamine substrate specificity, limiting the scope of its application. In this work, we designed a FRET-based ligation assay to monitor the bTG-mediated conjugation of the fluorescent proteins Clover and mRuby2. This assay allowed us to screen a library of random heptapeptide glutamine sequences for their reactivity with recombinant bTG in bacterial cells, using fluorescence assisted cell sorting. From this library, several reactive sequences were identified and kinetically characterized, with the most reactive sequence (YAHQAHY) having a kcat/KM value of 19 ± 3 μM-1 min-1. This sequence was then genetically appended onto a test protein as a reactive ‘Q-tag’ and fluorescently labelled with dansyl-cadaverine, in the first demonstration of protein labelling mediated by bTG.
Microbial natural products are specialized metabolites that have long been a rich source of human therapeutics. While the chemical diversity encoded in the genomes of microbes is believed to be large, the productivity of this modality has waned as traditional fermentation-based discovery methods have been plagued by high-rates of rediscovery, inefficient scaling, and incompatibility with target-based drug discovery. Here, we demonstrate a scalable discovery platform that couples dramatically improved assembly of deep-sequenced metagenomic samples with highly efficient, target-focused, in silica search strategies and synthetic biology to discover multiple novel inhibitors of human methionine aminopeptidase-1 (HsMetAP1), a validated oncology target. For one of these novel inhibitors, metapeptin B, we demonstrate sub-micromolar potency, strong selectivity for HsMetAP1 over HsMetAP2 and leverage natural congeners to rapidly elucidate key SAR elements. Our “next-gen” discovery platform overcomes many of the challenges constraining traditional methods, implies the existence of vast, untapped chemical diversity in nature, and demonstrates computationally-enabled precision discovery of modulators of human proteins of interest.
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