Bichitra Kumar Biswal

Membrane recruitment is a fundamental regulator of protein function. However, the allosteric mechanisms by which lipid binding controls protein activity remain poorly understood. In autophagy, the ubiquitin-like protein LC3 is lipid-anchored to autophagosomes, where it is essential for receptor recruitment and vesicle formation. While LC3-receptor interactions are structurally well defined, how membrane engagement governs LC3 functional dynamics has remained enigmatic. Here, we uncover that membrane binding triggers a major conformational transition in LC3, exposing functional pockets that are occluded in its cytosolic form. We demonstrate that this shift is mediated by dynamic coupling between the allosteric site (α3-loop5-β3-loop6) and the functional binding pockets. To conclusively test this mechanism, we utilised an ensemble-based protein design strategy guided by molecular dynamics to engineer the allosteric site. From a series of mutants, two variants emerged that stabilized LC3 conformation in either active or inactive state on the membrane. X-ray crystal structures of mutant LC3, biophysical assays, super-resolution microscopy, and TEM confirmed that the activated allosteric site mutant facilitates receptor binding and cargo capture. In contrast, the inactive variant is functionally inert on the membrane. Our work identifies a fundamental lipid-triggered allosteric site in LC3 that is critical for autophagy regulation and broader implications of membrane-dependent reprogrammable protein activities.

Mycobacterium tuberculosis (Mtb), the causative agent of human tuberculosis (TB), employs its de novo histidine (His) biosynthesis to escape host-inflicted His starvation. This makes the enzymes involved in this biosynthetic pathway promising anti-TB drug targets. In this study, employing the high-resolution crystal structure of imidazole glycerol phosphate dehydratase (IGPD) of the Mtb His pathway, three triazole scaffold molecules were identified as potential inhibitors of this enzyme. These high-resolution crystal structures of the enzyme-inhibitor complexes elucidated the key interactions responsible for their binding specificity and affinity. We also studied the interactions of these inhibitors with the enzyme at the atomic level and tested their cytotoxicity and efficacy in in vitro and in vivo models. Our findings revealed that the most prominent inhibitor, SF2, was safe in mice and effectively inhibited the in vitro growth of both free as well as in macrophage-internalized wild-type and drug-resistant Mtb clinical isolates. Notably, SF2 also showed a marginal reduction in the bacterial load in organs of mice infected with Mtb. Collectively, these findings advocate the chemical inhibition of IGPD of the His pathway as a novel anti-Mtb therapeutic strategy.

Unlike humans, Mycobacterium tuberculosis ( Mtb ), the causative agent of human tuberculosis, has a de novo histidine-biosynthesis pathway. The enzyme imidazole glycerol phosphate dehydratase (IGPD), which catalyses the conversion of imidazole glycerol phosphate to imidazole acetol phosphate, has been studied extensively from various organisms and has become a major target for the development of antibacterial, antiweed and antifungal small molecules. In our previous studies, we have shown that in crystals IGPD forms a 24-mer oligomeric state in which the monomers are arranged in 432 symmetry. In order to gain insights into the oligomeric state of Mtb IGPD in solution, we determined cryogenic sample electron microscopy (cryo-EM) structures of apo IGPD at 2.2 and 3.1 Å resolution. In addition, we also determined the cryo-EM structure of IGPD in the presence of 3-amino-1,2,4-triazole (ATZ) to 2.8 Å resolution. The results of this work, which corroborate those from the crystallographic studies, indicate that IGPD forms a homo-oligomeric structure in solution comprising of 24 subunits. ATZ binds in the active-site pocket of the enzyme, which is located at the interface of three monomers and tethers 24 ATZ molecules. The results of this study suggest that cryo-EM, in addition to being a rapidly evolving and complementary imaging technology for elucidating 3D structures of biological macromolecules, can be useful in pinpointing the mode of binding small molecules of low mass (here ∼85 Da) and mapping protein-ligand interactions, which could assist in the design of accurate (high-potency) inhibitors.

Enolase, a ubiquitous enzyme, catalyzes the reversible conversion of 2-phosphoglycerate (2PG) to phosphoenolpyruvate (PEP) in the glycolytic pathway of organisms of all three domains of life. The underlying mechanism of the 2PG to PEP conversion has been studied in great detail in previous work, however that of the reverse reaction remains to be explored. Here we present structural snapshots of Mycobacterium tuberculosis (Mtb) enolase in apo, PEP-bound and two 2PG-bound forms as it catalyzes the conversion of PEP to 2PG. The two 2PG-bound complex structures differed in the conformation of the bound product (2PG) viz the widely reported canonical conformation and a novel binding pose, which we refer to here as the alternate conformation. Notably, we observed two major differences compared with the forward reaction: the presence of Mg_B is non-obligatory for the reaction and 2PG assumes an alternate conformation that is likely to facilitate its dissociation from the active site. Molecular dynamics studies and binding free energy calculations further substantiate that the alternate conformation of 2PG causes distortions in both metal ion coordination and hydrogen-bonding interactions, resulting in an increased flexibility of the active-site loops and aiding product release. Taken together, this study presents a probable mechanism involved in PEP to 2PG catalysis that is likely to be mediated by the conformational change of 2PG at the active site.

To overcome the drug resistance crisis and shorten the current duration of human tuberculosis (TB) therapy, new anti-TB molecules is required. In an earlier study, we have shown that Mycobacterium tuberculosis ( Mtb ), the causative agent of TB, with a fractured de novo histidine biosynthesis fails to mount TB infection in mouse model, emboldening that disrupting the function of this pathway may constitute a novel strategy to curtailing TB infection. In this study, through a target based approach we have designed a number of triazole scaffold molecules specific to imidazole glycerol phosphate dehydratase (IGPD; HisB) of this pathway and have delineated atomic level interactions between the enzyme and inhibitors which pinpointed the specificity and the inhibitory mechanism. Importantly, these molecules exhibited significant potency against free as well as macrophage-internalized wild-type and drug-resistant clinical isolates in culture medium. Notably, a couple of these compounds showed efficacy in reducing the bacterial burden in Mtb -infected mouse model. The chemical inhibition of IGPD induces histidine auxotrophy in Mtb and brings in new prospects to the area of anti-TB drug discovery.

Aging is associated with changes in circulating levels of various molecules, some of which remain undefined. We find that concentrations of circulating taurine decline with aging in mice, monkeys, and humans. A reversal of this decline through taurine supplementation increased the health span (the period of healthy living) and life span in mice and health span in monkeys. Mechanistically, taurine reduced cellular senescence, protected against telomerase deficiency, suppressed mitochondrial dysfunction, decreased DNA damage, and attenuated inflammaging. In humans, lower taurine concentrations correlated with several age-related diseases and taurine concentrations increased after acute endurance exercise. Thus, taurine deficiency may be a driver of aging because its reversal increases health span in worms, rodents, and primates and life span in worms and rodents. Clinical trials in humans seem warranted to test whether taurine deficiency might drive aging in humans.

The unintended crystallization of proteins which generally originate from the expression host instead of the target recombinant proteins is periodically reported. Despite the massive technological advances in the field, assigning a structural model to the corresponding diffraction data is not a trivial task. Here, the structure of acyl-carrier protein synthase (AcpS) from Mycobacterium smegmatis (msAcpS), which crystallized inadvertently in an experimental setup to grow crystals of a Mycobacterium tuberculosis protein using M. smegmatis as an expression system, is reported. After numerous unsuccessful attempts to solve the structure of the target protein by the molecular-replacement method no convincing solutions were obtained, indicating that the diffraction data may correspond to a crystal of an artifactual protein, which was finally identified by the Sequence-Independent Molecular replacement Based on Available Databases (SIMBAD) server. The msAcpS structure was solved at 2.27 Å resolution and structural analysis showed an overall conserved fold. msAcpS formed a trimeric structure similar to those of other reported structures of AcpS from various organisms; however, the residues involved in trimer formation are not strictly conserved. An unrelated metal ion (Ni²⁺), which was possibly incorporated during protein purification, was observed in the proximity of His49 and His116. Structural and sequence differences were observed in the loop connecting the α3 and α4 helices that is responsible for the open and closed conformations of the enzyme. Moreover, the structural analysis of msAcpS augments the current understanding of this enzyme, which plays a crucial role in the functional activation of acyl-carrier proteins in the fatty-acid biosynthesis pathway.