Lymphocyte-specific protein tyrosine kinase (LCK) is essential for T cell antigen receptor (TCR)–mediated signal transduction. Here, we report two siblings homozygous for a novel LCK variant (c.1318C>T; P440S) characterized by T cell lymphopenia with skewed memory phenotype, infant-onset recurrent infections, failure to thrive, and protracted diarrhea. The patients’ T cells show residual TCR signal transduction and proliferation following anti-CD3/CD28 and phytohemagglutinin (PHA) stimulation. We demonstrate in mouse models that complete (Lck−/−) versus partial (LckP440S/P440S) loss-of-function LCK causes disease with differing phenotypes. While both Lck−/− and LckP440S/P440S mice exhibit arrested thymic T cell development and profound T cell lymphopenia, only LckP440S/P440S mice show residual T cell proliferation, cytokine production, and intestinal inflammation. Furthermore, the intestinal disease in the LckP440S/P440S mice is prevented by CD4+ T cell depletion or regulatory T cell transfer. These findings demonstrate that P440S LCK spares sufficient T cell function to allow the maturation of some conventional T cells but not regulatory T cells—leading to intestinal inflammation.
Enhanced green fluorescent protein (EGFP)-one of the most widely applied genetically encoded fluorescent probes-carries the threonine-tyrosine-glycine (TYG) chromophore. EGFP efficiently undergoes green-to-red oxidative photoconversion ("redding") with electron acceptors. Enhanced yellow fluorescent protein (EYFP), a close EGFP homologue (five amino acid substitutions), has a glycine-tyrosine-glycine (GYG) chromophore and is much less susceptible to redding, requiring halide ions in addition to the oxidants. In this contribution we aim to clarify the role of the first chromophore-forming amino acid in photoinduced behavior of these fluorescent proteins. To that end, we compared photobleaching and redding kinetics of EGFP, EYFP, and their mutants with reciprocally substituted chromophore residues, EGFP-T65G and EYFP-G65T. Measurements showed that T65G mutation significantly increases EGFP photostability and inhibits its excited-state oxidation efficiency. Remarkably, while EYFP-G65T demonstrated highly increased spectral sensitivity to chloride, it is also able to undergo redding chloride-independently. Atomistic calculations reveal that the GYG chromophore has an increased flexibility, which facilitates radiationless relaxation leading to the reduced fluorescence quantum yield in the T65G mutant. The GYG chromophore also has larger oscillator strength as compared to TYG, which leads to a shorter radiative lifetime (i.e., a faster rate of fluorescence). The faster fluorescence rate partially compensates for the loss of quantum efficiency due to radiationless relaxation. The shorter excited-state lifetime of the GYG chromophore is responsible for its increased photostability and resistance to redding. In EYFP and EYFP-G65T, the chromophore is stabilized by π-stacking with Tyr203, which suppresses its twisting motions relative to EGFP.
Aryl alcohol-type or phenolic fluorophores offer diverse opportunities for developing bioimaging agents and fluorescence probes. Due to the inherently acidic hydroxyl functionality, phenolic fluorophores provide pH-dependent emission signals. Therefore, except for developing pH probes, the pH-dependent nature of phenolic fluorophores should be considered in bioimaging applications but has been neglected. Here we show that a simple structural remedy converts conventional phenolic fluorophores into pH-resistant derivatives, which also offer "medium-resistant" emission properties. The structural modification involves a single-step introduction of a hydrogen-bonding acceptor such as morpholine nearby the phenolic hydroxyl group, which also leads to emission bathochromic shift, increased Stokes shift, enhanced photo-stability and stronger emission for several dyes. The strategy greatly expands the current fluorophores' repertoire for reliable bioimaging applications, as demonstrated here with ratiometric imaging of cells and tissues.
Thiotemplated pyrrole is a prevailing intermediate in the synthesis of numerous natural products where the pyrrole is tethered to a carrier protein (CP). Biosynthesis of the pyrrole requires oxidation of an L-proline side chain. Herein, we investigated the biocatalytic mechanism of proline-to-pyrrole synthesis using the recently reported (Thapa et al., Biochemistry, 2019, 58, 918) structure of a Type II non-ribosomal protein synthetase (NRPS) Bmp3-Bmp1 (Oxidase-CP) complex. The substrate (L-proline side chain) is attached to the Bmp1(CP) and the catalytic site is located inside the flavin-dependent oxidase (Bmp3). Interestingly, the FAD molecule is free (unbound) within the Bmp3 catalytic site. We show that the FAD isoalloxazine ring is stabilized in the catalytic site of Bmp3 by strong hydrogen bonding with Asn123, Ile125, Ser126, and Thr158. The stability of the dimeric Bmp3 system including one FAD molecule in our simulation suggests that the tetrameric Bmp3 assembly, as found in the crystal structure, is not functionally important. After the initial deprotonation followed by an enamine-imine tautomerization, oxidation of either the C2-C3 bond or the C2-N1 bond, through a hydride transfer (either from C3 or N1), is required for the pyrrole synthesis. Using molecular dynamics simulations, quantum mechanics/molecular mechanics simulations, and electronic structure calculations, we conclude that the hydride transfer is more likely to occur from C3 than N1. Additionally, we demonstrated the elasticity in the oxidase active site through enzymatic synthesis of proline derivatives.
These are all input and final structures for a set of simulations investigating the protein MmpL3. All topology, parameter, and NAMD configuration files for running the molecular dynamics simulations are also included. Recommended software to use is Visual Molecular Dynamics (VMD).
These are all input and final structures for a set of simulations investigating the protein MmpL3. All topology, parameter, and NAMD configuration files for running the molecular dynamics simulations are also included. Recommended software to use is Visual Molecular Dynamics (VMD).
Photoinduced electron transfer in fluorescent proteins from the GFP family can be regarded either as an asset facilitating new applications or as a nuisance leading to the loss of optical output. Photooxidation commonly results in green-to-red photoconversion called oxidative redding. We discovered that yellow FPs do not undergo redding; however, the redding is restored upon halide binding. Calculations of the energetics of one-electron oxidation and possible electron transfer (ET) pathways suggested that excited-state ET proceeds through a hopping mechanism via Tyr145. In YFPs, the π-stacking of the chromophore with Tyr203 reduces its electron-donating ability, which can be restored by halide binding. Point mutations confirmed that Tyr145 is a key residue controlling ET. Substitution of Tyr145 by less-efficient electron acceptors resulted in highly photostable mutants. This strategy (i.e., calculation and disruption of ET pathways by mutations) may represent a new approach toward enhancing photostability of FPs.
