Cryo-EM structures of ClC-2 chloride channel reveal the blocking mechanism of its specific inhibitor AK-42.

ClC-2 transports chloride ions across plasma membranes and plays critical roles in cellular homeostasis. Its dysfunction is involved in diseases including leukodystrophy and primary aldosteronism. AK-42 was recently reported as a specific inhibitor of ClC-2. However, experimental structures are still missing to decipher its inhibition mechanism. Here, we present cryo-EM structures of apo ClC-2 and its complex with AK-42, both at 3.5 Å resolution. Residues S162, E205 and Y553 are involved in chloride binding and contribute to the ion selectivity. The side-chain of the gating glutamate E205 occupies the putative central chloride-binding site, indicating that our structure represents a closed state. Structural analysis, molecular dynamics and electrophysiological recordings identify key residues to interact with AK-42. Several AK-42 interacting residues are present in ClC-2 but not in other ClCs, providing a possible explanation for AK-42 specificity. Taken together, our results experimentally reveal the potential inhibition mechanism of ClC-2 inhibitor AK-42.

RNA m6A methylation regulates dissemination of cancer cells by modulating expression and membrane localization of ß-catenin.

N6-methyladenosine (m6A) methylation, which is modified by the METTL3/METTL14 complex, is a dominant internal modification in mammalian RNA and tightly linked to cancer progression. Here we reveal that METTL3-promoted cell migration, invasion, and epithelial-to-mesenchymal transition (EMT) are associated with expression and membrane localization of ß-catenin (encoded by CTNNB1), as opposed to Wnt signaling activation in various types of cancer cells, including cervical, lung, and liver cancer. Specifically, METTL3 regulates the transcription, mRNA decay, translation, and subcellular localization of ß-catenin. For CTNNB1 expression, METTL3 indirectly suppresses CTNNB1 transcription by stabilizing its transcription suppressor E2F1 mRNA； deposition of 5'UTR m6A in CTNNB1 promotes its decay in a content-dependent manner via YTHDF2 recognition; 5'UTR m6A in CTNNB1 suppresses its translation efficiency, whereas the global METTL3 level controls the canonical and non-canonical translation of CTNNB1, which is probably associated with the interaction between YTHDF1 and eIF4E1/eIF4E3. For ß-catenin translocation, METTL3 represses membrane localization of ß-catenin and its interaction with E-cadherin by downregulating c-Met kinase, leading to inhibition of cell motility. In vitro, in vivo, and clinical analyses confirm the essential role of ß-catenin and its expression regulators in cancer cell dissemination. The findings not only expand our understanding of m6A modification and its roles in gene expression and subcellular localization of targets but also suggest that the METTL3/ß-catenin axis might be a potential target to inhibit cancer metastasis.

Modular enzyme assembly for enhanced cascade biocatalysis and metabolic flux.

Enzymatic reactions in living cells are highly dynamic but simultaneously tightly regulated. Enzyme engineers seek to construct multienzyme complexes to prevent intermediate diffusion, to improve product yield, and to control the flux of metabolites. Here we choose a pair of short peptide tags (RIAD and RIDD) to create scaffold-free enzyme assemblies to achieve these goals. In vitro, assembling enzymes in the menaquinone biosynthetic pathway through RIAD-RIDD interaction yields protein nanoparticles with varying stoichiometries, sizes, geometries, and catalytic efficiency. In Escherichia coli, assembling the last enzyme of the upstream mevalonate pathway with the first enzyme of the downstream carotenoid pathway leads to the formation of a pathway node, which increases carotenoid production by 5.7 folds. The same strategy results in a 58% increase in lycopene production in engineered Saccharomyces cerevisiae. This work presents a simple strategy to impose metabolic control in biosynthetic microbe factories.

Synthetic Multienzyme Complexes, Catalytic Nanomachineries for Cascade Biosynthesis In Vivo.

Multienzyme complexes, or metabolons, are assemblies or clusters of sequential enzymes that naturally exist in metabolic pathways. These nanomachineries catalyze the conversion of metabolites more effectively than the freely floating enzymes by minimizing the diffusion of intermediates in vivo. Bioengineers have devised synthetic versions of multienzyme complexes in cells to synergize heterologous biosynthesis, to improve intracellular metabolic flux, and to achieve higher titer of valuable chemical products. Here, we utilized orthogonal protein reactions (SpyCatcher/SpyTag and SnoopCatcher/SnoopTag pairs) to covalently assemble three key enzymes in the mevalonate biosynthesis pathway and showed 5-fold increase of lycopene and 2-fold increase of astaxanthin production in Escherichia coli. The multienzyme complexes are ellipsoidal nanostructures with hollow interior space and uniform thickness and shapes. Intracellular covalent enzyme assembly has yielded catalytic nanomachineries that drastically enlarged the flux of carotenoid biosynthesis in vivo. These studies also deepened our understanding on the complexity of hierarchical enzyme assembly in vivo.