research highlights

UNFOLDED PROTEIN RESPONSE electrophoresis aided by a clickable version of P7C3, the authors identified NAMPT Letting go of stress Cell 158, 1362–1374 (2014) as the compound’s target. NAMPT is the rate-limiting enzyme in the NAD salvage The unfolded protein response (UPR) pathway helps cells counteract stress that pathway, and dox depletes cellular NAD perturbs folding of proteins in the endoplasmic reticulum (ER). UPR resets proteostasis levels, so discovering NAMPT’s role as by lowering protein flux into the ER through translational downregulation or mRNA the P7C3 target could explain its ability to turnover and by elevating the expression of ER proteins that manage accumulated protect against dox-mediated toxicity if it unfolded proteins. Reid et al. now report a new pathway that limits protein transit into is able to enhance NAMPT activity. Indeed, the ER by dynamic relocalization of mRNA and ribosomes to the cytoplasm. Using the authors found that P7C3 could activate thapsigargin and dithiothreitol as reagents to induce protein folding stress in cells, NAMPT and replenish NAD levels depleted the authors assessed the location and activity of translation over time using ribosome by dox. There was a striking correlation profiling and RNA-seq. Their analysis suggests that early UPR focuses on lowering between the ability of 30 P7C3 variants to translational efficiency—particularly for the subset of ER-tethered polyribosomes that activate NAMPT in vitro, protect neurons are translating mRNAs for membrane and secretory proteins—in a rapid process that from dox toxicity, compete with photo- involves selective release of these mRNA–ribosome complexes into the cytoplasm, cross-linking by an active P7C3 variant where they may continue translation. This pathway is also reversible, as kinetic analysis and restore NAD levels in dox-treated revealed that this translating pool of mRNAs becomes relocalized to the ER membrane cells, validating NAMPT as the efficacy upon removal of folding stress. Though additional studies will be needed to identify target and implicating it in regulating the molecular interactions that regulate ribosome tethering and release at the ER, the neurogenesis. MB current model provides a new mechanism for reducing protein flux stress at the ER during the early stages of UPR. TLS PROTEIN TURNOVER Mitochondrial immaturity CARBOHYDRATES Cell Metab. doi:10.1016/j.cmet.2014.07.024 role that can be played by the cellobiose Cutting out starch dehydrogenase known to pair with Mitochondrial proteins synthesized in the Proc. Natl. Acad. Sci. USA 111, 13822–13827 cellulose-degrading PMOs, though cytosol contain a cleavable N-terminal (2014) whether this partnership occurs in vivo presequence that directs them to remains unknown. The enzyme binds and their destination. Upon arrival in the is dependent for activity on a single copper mitochondrion, the presequence peptide is atom, and spectroscopic data support a removed by the processing peptidase MPP metal binding site consistent with those and subsequently degraded by the matrix of other PMOs. These results provide an peptidasome Cym1 (also known as PreP). important expansion of the PMO family In the mitochondria of yeast cym1 mutants, and offer a new starch-degrading Mossmann et al. detected the accumulation enzyme for potential industrial of proteins containing the N-terminal Nature America, Inc. All rights reserved. America, Inc. © 201 4 Nature Polysaccharide monooxygenases (PMOs) applications. CG presequence, suggesting that MPP activity are Cu-dependent enzymes that cleave was dependent on peptide degradation. carbohydrate polymers including chitin, TARGET IDENTIFICATION The unprocessed proteins were identified as npg cellulose and hemicellulose. The structures being required for mitochondrial processes of these enzymes display several conserved NAD salvages neurons such as ATP synthesis and respiration, Cell , 1324–1334 (2014) elements, including a Cu-coordinating 158 which correlated with the defects observed ‘histidine brace’. However, their sequence Despite the importance of protecting in cym1 mutants, such as increased reactive homology is low, making discovery of newborn and adult hippocampal neurons oxygen species and decreased membrane new family members difficult. Vu et al. from death, there are no therapeutic potential. Interestingly, Cym1 was now use the signal peptide associated strategies available. A multiyear screening previously shown to degrade the amyloid-β with these secreted proteins along with approach aimed at identifying compounds peptide (Aβ), and the mitochondrial the strict requirement for an N-terminal that enhance hippocampal neurogenesis defects observed in cym1 mutants histidine—part of the histidine brace—to in adult mice identified a lead scaffold, resembled features observed in Alzheimer’s search Neurospora crassa for putative P7C3. P7C3 analogs mitigate the normal disease (AD) patients. The introduction PMOs. The authors found 21 such proteins turnover of newborn neurons. To of Aβ produced the same feedback whose sequences also contained the second determine the molecular target of P7C3, effects on MPP activity as cym1 mutants residue of the histidine brace and a known Wang et al. first developed a cell-based with the accumulation of unprocessed active site motif. NCU08746 was notable in assay that reports on P7C3 protection from mitochondrial proteins. Finally, the containing a C-terminal domain related to toxicity. The authors found that among maturation of mitochondrial preproteins the family 20 carbohydrate-binding module the eight different toxins and hundreds of was also inhibited in mouse and human (CBM20), also known as the starch-binding proneurogenic compounds they tested, only AD brain samples, suggesting a potentially domain, and so was predicted to bind the P7C3 scaffold compounds protected conserved mechanism to explain the amylose, a substrate not yet represented cultured cells from apoptosis induced by effects of Aβ peptide on mitochondrial among PMOs. In vitro characterization doxorubicin (dox) and that this activity function. GM demonstrated that NCU08746 can cleave correlated with the proneurogenic activity several starch substrates. This activity is of P7C3 in living mice. Using a photo-cross- Written by Mirella Bucci, Joshua M. Finkelstein, dependent on oxygen and a reductant, a linking approach and two-dimensional gel Catherine Goodman, Grant Miura & Terry L. Sheppard

NATURE CHEMICAL BIOLOGY | VOL 10 | NOVEMBER 2014 | www.nature.com/naturechemicalbiology 877