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PROMISCUITY OF MULTIDRUG RESISTANCE PROTEIN 1 SUBSTRATE BINDING EXPLAINED
ALLOSTERIC INHIBITOR ENABLES DUAL ATTACK ON CHRONIC MYELOID LEUKEMIA
Reprinted from Cell, 168, Johnson, Z. L., and Chen, J., Structural Basis of Substrate Recognition by the Multidrug Resistance Protein MRP1, 1075−1085. Copyright 2017, with permission from Elsevier.
Cells in many tissue types receive an endless barrage of bloodborne substances that are either produced within the body (i.e., endobiotics) or derived from external sources (i.e., xenobiotics). Many substances can be actively or passively transported into the cell, where useful molecules are retained, and nonuseful or potentially harmful substances are shuttled back out of the cell through dedicated exporter proteins. This filtering process is essential to healthy cell maintenance but can spell disaster when cells kick out drug molecules intended to improve cell function or, in the case of anticancer chemotherapy, trigger apoptosis. One of the exporter proteins frequently implicated in drug resistance, Multidrug Resistance Protein 1 (MRP1), has recently been structurally characterized by Zachary Lee Johnson and Jue Chen at the Rockefeller University (Cell, 2017, 168, 1075−1085). The research duo obtained cryoelectron microscopy images of both free bovine MRP1 as well as MRP1 bound to one of its endogenous substrates, leukotriene C4 (LTC4). These images provided clarification of a long-standing puzzle relating to MRP1 function: although most proteins process very specific substrates, MRP1 is able to recognize and export a wide variety of structurally dissimilar molecules. The structural images revealed that MRP1 has a large substrate binding site containing a positively charged pocket facing a hydrophobic pocket, which are capable of stabilizing ligands through hydrogen bonding or van der Waals interactions, respectively. The authors speculate that the bipartite nature of the binding site, which had decades ago been predicted to exist but had not been previously observed, explains MRP1’s preference for exporting amphipathic organic acids. The authors also note that substrate binding induces a conformational change that primes MRP1 for ATP hydrolysis, which in turn powers its transporter activity. Heidi A. Dahlmann © 2017 American Chemical Society
Reprinted by permission from Macmillan Publishers Ltd.: Nature, Wylie, A. A., et al., 543, 733−737, DOI: 10.1038/ nature21702, copyright 2017.
Chronic myeloid leukemia (CML), which accounts for up to a quarter of leukemia occurrences in Western populations, is the first type of cancer for which a specific chromosomal abnormality was characterized. In particular, a portion of human chromosome 22 translocates to chromosome 9, in the process fusing chromosome 9’s gene for tyrosine kinase ABL1 onto chromosome 22’s gene for breakpoint cluster region (BCR) protein. The resulting fusion protein (BCR-ABL1) is a constitutively active tyrosine kinase that enables uncontrollable cell proliferation, which consequently facilitates carcinogenesis. Due to BCR-ABL1 protein’s role in the progression of CML, it is naturally an attractive chemotherapy target. ABL1 kinase inhibitors have become a mainstay of CML treatment therapy; in fact, one analysis indicated that the overall survival of patients achieving complete cytogenic remission after treatment with the active-site ABL1 inhibitor imatinib was found to not be statistically significantly different from that of the general population (J. Natl. Cancer Inst. 2011, 103, 553−561). As is the case with virtually all chemotherapies, side effects and the development of drug-resistant cells can hamper ABL1 inhibitor effectiveness. However, combination therapy with drugs that have different modes of action allows doses to be reduced to mitigate side effects and significantly slows the development of drug resistance. One drug that could potentially complement active-site ABL1 inhibitors is asciminib (ABL001), an allosteric-site inhibitor that Published: April 21, 2017 883
DOI: 10.1021/acschembio.7b00294 ACS Chem. Biol. 2017, 12, 883−885
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a highly conjugated side chain, or “molecular wire,” that penetrates into the cell membrane. At resting potential or during hyperpolarization, photoinduced electron transfer (PeT) through the molecular wire quenches the fluorescence signal. In contrast, during neuronal firing, PeT is suppressed, creating a bright spike in the fluorescence signal. The research team demonstrated that RVF5 could be used to track rapid voltage changes within individual cells in both cultured neurons and in intact mouse brain tissue samples. Heidi A. Dahlmann
was recently characterized by Andrew A. Wylie and co-workers (Nature 2017, DOI: 10.1038/nature21702). Having determined that cultured CML cells treated with ABL001 acquired drug resistance mutations that were distinctly different than those induced by active-site inhibitor treatment, the research team went on to demonstrate that model mice treated with either type of drug alone acquired resistance, but those treated with ABL001 in combination with an active-site inhibitor showed complete durable tumor regression. ABL001 is now in phase 1 clinical trials as a candidate for CML combination therapy. Heidi A. Dahlmann
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FINDING NEW ENZYMES IN THE MICROBIOME Rhamnogalacturonan-II, or RG-II, is a highly complex pectic polysaccharide found in plants. The structure of this glycan is elaborate, with an array of 13 different sugars and 21 glycosidic linkages, yet it is almost completely digested when consumed as part of the human diet. Prior work showed that RG-II and other plant cell wall glycans feed the gut microbiota, but understanding exactly which bacteria are responsible for breaking them down and the set of enzymes that they bring to the task remains mysterious. Now, Ndeh et al. (Nature 2017, DOI: 10.1038/nature21725) take a hint from prior work suggesting that bacteria of the Bacteriodes genus can at least partially break down this complex glycan. The researchers found that about 30% of the tested strains could grow on an RG-II medium before honing in on the human microbiome bacterium Bacteroides thetaiotaomicron for further characterization. Analysis of its metabolites showed that this species could cleave 20 of the 21 bonds in the glycan. Further analysis of genes upregulated by growth on RG-II uncovered seven new families of glycoside hydrolase enzymes. A pathway to complete RG-II depolymerization was then mapped out using in vitro digestion reactions or by observing changes in the intermediates found in mutant strains. Many of the glycosidic bonds had no previously associated enzyme to break them, so this study moved several genes previously trapped in the purgatory of “hypothetical protein” annotation into their rightful place as hydrolases or esterases of the degradome. The new mechanistic information also revealed that prior models for the RG-II structure were partially incorrect, and the glycan’s precise stereochemistry needed revision. This study indicates that many secrets are still hidden in the human microbiome with new enzymatic activities or targets remaining to be discovered. Jason G. Underwood
VOLTAGE-SENSITIVE DYE ILLUMINATES NEURONAL ACTIVITY
Kulkarni, R. U., et al. Proc. Natl. Acad. Sci., U. S. A., 114, 2813− 2818. Copyright 2017 National Academy of Sciences, U. S. A.
Sensory perception, motor control, hormone releasethese seemingly disparate biological functions, among others, are all dependent on the activity of neurons, which are specialized cells that initiate information transfer through electrical signals. Like all eukaryotic cells, neurons are bound by lipid bilayer membranes that act as insulators to maintain a difference in charge across the inside and outside of the cells. When a neuron receives a chemical cue to propagate a signal, specialized protein channels open to allow ions to flow into or out of the cell, thereby altering its membrane potential. Depolarization occurs when the interior voltage becomes less negative, while hyperpolarization occurs when the interior voltage becomes more negative. When a neuron “fires,” it undergoes a very rapid depolarization followed by a hyperpolarization recovery phase, triggering the timed release of neurotransmitters that induce neighboring neurons to fire, thus propagating electrical signals across neuronal networks. Recently, a research team led by Evan W. Miller has reported the development of a voltage-sensitive dye, dubbed “RVF5,” that allows imaging of firing across neuronal networks with high spatiotemporal resolution (Proc. Natl. Acad. Sci., U. S. A. 2017, 114, 2813−2818). RVF5 consists of a rhodol-based fluorophore that can be excited with either visible or infrared light linked to
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ODD-CARBON DIACID BIOSYNTHESIS
Reprinted with permission from Haushalter, R.W., et al., J. Am. Chem. Soc., DOI: 10.1021/jacs.6b11895. Copyright 2017 American Chemical Society
Dicarboxylic acids (DCAs) are critical building blocks for a host of consumer products: plastics, polyesters, medications, and fragrances. But many of these compounds come from petroleum-based sources, so researchers have been looking for 884
DOI: 10.1021/acschembio.7b00294 ACS Chem. Biol. 2017, 12, 883−885
ACS Chemical Biology
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biological routes to synthesize them. Pathways have been established for synthesizing compounds with even numbers of carbon atoms, but pathways for those with odd numbers of carbons have remained elusive. Now, Haushalter et al. report a novel pathway for the production of odd-carbon DCAs in E. coli (J. Am. Chem. Soc. 2017, DOI: 10.1021/jacs.6b11895). To reach this goal, the team focused on the biotin biosynthetic pathway, which includes a seven-carbon DCA intermediate. During natural biotin biosynthesis, BioC diverts a small amount of malonyl-acyl carrier protein (ACP) from normal cellular fatty acid production. BioC methylates this even-numbered carbon skeleton. Then further additions of malonyl ACP produce the seven-carbon intermediate, which is hydrolyzed and released via BioH. In initial tests that overexpressed BioC from Bacillus cereus in E. coli, another team observed growth inhibition in their cultures, which suggested a restriction of normal fatty acid biosynthesis in these cells. To counteract this problem, this team coexpressed a truncated version of TesA, an E. coli esterase, to release their DCA products from cellular ACP and allow these proteins to resume their normal function in fatty acid biosynthesis. That combination did not show growth inhibition, and the team used LC-MS to analyze the products. The cells produced a mixture of odd-carbon DCAs ranging from 9 to 15 carbons in length. Brassylic acid (a product with 13 carbons) predominated, representing up to 90% of the DCA product. The team also examined BioC from other bacterial strains and other strategies for boosting yields. Deleting BioH increased DCA yieldand the products were still predominantly diacids which had been hydrolyzed by other means. The team also looked at BioC enzymes from other bacterial strains in combination with TesA. Kurthia BioC showed modest production gains (∼20%) over B. cereus and confirmed that this pathway diverts carbon building blocks away from fatty acid biosynthesis. This novel pathway could prove useful for producing a variety of substances from renewable sources, but further work will be needed to boost DCA yield and manage the trade-off between DCA production and natural fatty acid synthesis. Sarah A. Webb
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DOI: 10.1021/acschembio.7b00294 ACS Chem. Biol. 2017, 12, 883−885
