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BIOCONJUGATION insensitive to ReACT, the probe was applied transcripts along with developmental to selectively label reactive methionines for delay and cell cycle progression defects, Methionine’s time to shine ­­activity-based protein profiling, leading to indicative of a defective MZT. Overall, these Science 355, 597–602 (2017) the identification of hyperreactive residues findings indicate that the m6A modification in enolase that have redox-active functions has a key role in transcriptome switching in vivo. With methionine now a viable choice during early embryo development. GM for chemoselective bioorthogonal tagging, the options for bioconjugation are wider IMAGING AAAS than ever. CD Luciferase matchmaker DEVELOPMENT J. Am. Chem. Soc. 139, 2351–2358 (2017)

Methionine is one of the rarest amino Marking the transition Nature , 475–478 (2017) acids in proteins, making it a potentially 542 attractive handle for highly selective protein N6-Methyladenosine (m6A) is an mRNA modification. However, because of a lack post-translational modification that is of robust bioorthogonal reactions for recognized by the RNA-binding protein targeting methionine, it has historically YTHDF2, which regulates mRNA stability. JACS been overlooked in favor of the other In order to understand the dynamics of sulfur-containing residue, cysteine, which m6A modification during embryonic is more nucleophilic. Lin et al. have now development, Zhao et al. performed developed a modification strategy that m6A sequencing of zebrafish maternal utilizes methionine’s redox activity for transcripts and found that 36% were conjugation under physiological conditions. m6A modified. Given that these maternal Bioluminescence imaging techniques This method, redox-activated chemical transcripts decreased in abundance during use luciferase to catalyze the oxidation tagging (ReACT), relies on an oxaziridine the maternal–zygotic transition (MZT), of a luciferin substrate, producing light. reagent that converts the methionine side with a corresponding increase in zygotic While natural and artificial variants of chain to a sulfimide conjugation product transcription, the authors hypothesized that both enzyme and substrate enable imaging while reducing the reagent to an aldehyde. YTHDF2 might be involved in maternal with different wavelengths of light, the This reaction is selective for methionine mRNA clearance. Injection of GFP- poor selectivity of luciferases for their over other nucleophilic amino acids labeled mRNA with and without the m6A cognate luciferins (over other structurally and does not denature proteins, and the modification into maternal ythdf2 zebrafish similar luciferins) hampers their use in product is stable when exposed to reducing mutants revealed that m6A-modified RNA multicomponent imaging. To address this reagents or elevated temperatures for up exhibited a slower rate of degradation shortcoming, Jones et al. targeted both sides to one hour. To demonstrate its utility, during the MZT in these mutants relative of the luciferase–luciferin pair to engineer the authors applied the ReACT method to wild-type embryos. Consistent with this orthogonality. First, the authors designed to label proteins with alkyne- or azide- result, maternal ythdf2 zebrafish mutants and synthesized a panel of luciferin analogs containing tags for further click chemistry exhibited an upregulation of maternal with increased steric bulk that weakened and to synthesize antibody–drug conjugates. transcripts involved in cell cycle and their binding to the wild-type luciferase Furthermore, as methionine sulfoxide is reproduction and lowered levels of zygotic while retaining their intrinsic potential for light emission. The authors then generated libraries of luciferase variants by targeted DRUG DISCOVERY mutagenesis at residues predicted to be PLoS Pathog. doi:10.1371/ important for substrate binding. Screening Uncoupling coupled transport journal.ppat.1006180 (2017) this mutant library for enzymes that were active with luciferin analogs also enabled The human malaria parasite Plasmodium falciparum has developed resistance to most iterative rounds of library diversification, and a secondary screen rated all of the

© 2017 Nature America, Inc., part of Springer Nature. All rights reserved. All rights part Nature. of Springer Inc., America, Nature © 2017 existing drugs, so drugs with new mechanisms of action are urgently needed. One parasite pathway that is underexplored as a potential therapeutic target is that involved pairwise combinations of enzyme and in the generation of ATP. The parasite takes up glucose and converts it via glycolysis substrate for orthogonality. From these to lactate, generating ATP in the process. The lactate end product is exported from the screens, three luciferases were identified parasite via a proton-coupled (pH-dependent) transporter. Hapuarachchi et al. screened that were capable of orthogonal activation a curated collection of antiplasmodial compounds for their effect on parasite pH and by select luciferin analogs in vitro and in identified 15 compounds that acidified the parasite. Monitoring the acidification under cell culture. These new luciferase–luciferin conditions in which glycolysis was either active or turned off revealed that 2 of the 15 pairs are themselves potentially useful hits exerted their effect on pH through the inhibition of proton-coupled lactate efflux. contributions to the bioluminescence Parasites selected for resistance to one of these two compounds that were also resistant toolbox, and this approach to their to the otheR had a mutation in the lactate–H+ transporter PfFNT. The compounds generation could be applied to future efforts inhibited lactate transport both in isolated parasites and in Xenopus oocytes expressing in developing selective imaging probes. CD the PfFNT transporter. The resistance-associated mutation in PfFNT reduced inhibition of transport in both systems. This study highlights the potential of the lactate transporter PfFNT as an antimalarial drug target. MB Written by Mirella Bucci, Caitlin Deane and Grant Miura

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