Editing of Misaminoacylated tRNA Controls the Sensitivity of Amino Acid Stress Responses in Saccharomyces cerevisiae

Amino acid starvation activates the protein kinase Gcn2p, leading to changes in gene expression and translation. Gcn2p is activated by deacylated tRNA, which accumulates when tRNA aminoacylation is limited by lack of substrates or inhibition of synthesis. Pairing of amino acids and deacylated tRNAs is catalyzed by aminoacyl-tRNA synthetases, which use quality control pathways to maintain substrate specificity. Phenylalanyl-tRNA synthetase (PheRS) maintains specificity via an editing pathway that targets non-cognate Tyr-tRNAPhe. While the primary role of aaRS editing is to prevent misaminoacylation, we demonstrate editing of misaminoacylated tRNA is also required for detection of amino acid starvation by Gcn2p. Ablation of PheRS editing caused accumulation of Tyr-tRNAPhe (5%), but not deacylated tRNAPhe during amino acid starvation, limiting Gcn2p kinase activity and suppressing Gcn4p-dependent gene expression. While the PheRS-editing ablated strain grew 50% slower and displayed a 27-fold increase in the rate of mistranslation of Phe codons as Tyr compared to wild type, the increase in mistranslation was insufficient to activate an unfolded protein stress response. These findings show that during amino acid starvation a primary role of aaRS quality control is to help the cell mount an effective stress response, independent of the role of editing in maintaining translational accuracy

Mechanisms involved in AMPK-mediated deposition of tight junction components to the plasma membrane.

AMP activated protein kinase (AMPK) activation promotes early stages of epithelial junction assembly. AMPK activation in MDCK renal epithelial cells facilitates localization of the junction-associated proteins aPKCzeta and Par3 to the plasma membrane and promotes conversion of Cdc42, a key regulator of epithelial polarization and junction assembly, to its active GTP bound state. Furthermore, Par3 is an important regulator of AMPK-mediated aPKCzeta localization. Both aPKCzeta and Par3 serve as intermediates in AMPK-mediated junction assembly, with inhibition of aPKCzeta activity or Par3 knockdown disrupting AMPK's ability to facilitate zonula occludens (ZO-1) localization. AMPK phosphorylates the adherens junction protein afadin and regulates its interaction with the tight junction protein zonula occludens (ZO)-1. Afadin is phosphorylated at two critical sites, S182 (residing within an aPKCzeta consensus site) and S1049 (residing within an AMPK consensus site), that are differentially regulated during junction assembly and that exert different effects on the process. Expression of phospho-defective mutants (S182A and S1082A) perturbed ZO-1 localization to the plasma membrane during AMPK-induced junction assembly. Expression of S182A increased the ZO-1/afadin interaction, while S1049A reduced this interaction during extracellular calcium-induced junction assembly. Inhibition of aPKCzeta activity also increased the ZO-1/afadin interaction. Taken together, these data suggest that aPKCzeta phosphorylation of afadin terminates the ZO-1/afadin interaction, and thus permits the later stages of junction assembly

Designed Phosphoprotein Recognition in <i>Escherichia coli</i>

Protein phosphorylation is a central biological mechanism for cellular adaptation to environmental changes. Dysregulation of phosphorylation signaling is implicated in a wide variety of diseases. Thus, the ability to detect and quantify protein phosphorylation is highly desirable for both diagnostic and research applications. Here we present a general strategy for detecting phosphopeptide–protein interactions in <i>Escherichia coli</i>. We first redesign a model tetratricopeptide repeat (TPR) protein to recognize phosphoserine in a sequence-specific fashion and characterize the interaction with its target phosphopeptide <i>in vitro</i>. We then combine <i>in vivo</i> site-specific incorporation of phosphoserine with split mCherry assembly to observe the designed phosphopeptide–protein interaction specificity in <i>E. coli</i>. This <i>in vivo</i> strategy for detecting and characterizing phosphopeptide–protein interactions has numerous potential applications for the study of natural interactions and the design of novel ones

Designed Phosphoprotein Recognition in <i>Escherichia coli</i>

Protein phosphorylation is a central biological mechanism for cellular adaptation to environmental changes. Dysregulation of phosphorylation signaling is implicated in a wide variety of diseases. Thus, the ability to detect and quantify protein phosphorylation is highly desirable for both diagnostic and research applications. Here we present a general strategy for detecting phosphopeptide–protein interactions in <i>Escherichia coli</i>. We first redesign a model tetratricopeptide repeat (TPR) protein to recognize phosphoserine in a sequence-specific fashion and characterize the interaction with its target phosphopeptide <i>in vitro</i>. We then combine <i>in vivo</i> site-specific incorporation of phosphoserine with split mCherry assembly to observe the designed phosphopeptide–protein interaction specificity in <i>E. coli</i>. This <i>in vivo</i> strategy for detecting and characterizing phosphopeptide–protein interactions has numerous potential applications for the study of natural interactions and the design of novel ones

Designed Phosphoprotein Recognition in <i>Escherichia coli</i>

Protein phosphorylation is a central biological mechanism for cellular adaptation to environmental changes. Dysregulation of phosphorylation signaling is implicated in a wide variety of diseases. Thus, the ability to detect and quantify protein phosphorylation is highly desirable for both diagnostic and research applications. Here we present a general strategy for detecting phosphopeptide–protein interactions in <i>Escherichia coli</i>. We first redesign a model tetratricopeptide repeat (TPR) protein to recognize phosphoserine in a sequence-specific fashion and characterize the interaction with its target phosphopeptide <i>in vitro</i>. We then combine <i>in vivo</i> site-specific incorporation of phosphoserine with split mCherry assembly to observe the designed phosphopeptide–protein interaction specificity in <i>E. coli</i>. This <i>in vivo</i> strategy for detecting and characterizing phosphopeptide–protein interactions has numerous potential applications for the study of natural interactions and the design of novel ones

Comparative Proteomics Enables Identification of Nonannotated Cold Shock Proteins in <i>E. coli</i>

Recent advances in mass spectrometry-based proteomics have revealed translation of previously nonannotated microproteins from thousands of small open reading frames (smORFs) in prokaryotic and eukaryotic genomes. Facile methods to determine cellular functions of these newly discovered microproteins are now needed. Here, we couple semiquantitative comparative proteomics with whole-genome database searching to identify two nonannotated, homologous cold shock-regulated microproteins in Escherichia coli K12 substr. MG1655, as well as two additional constitutively expressed microproteins. We apply molecular genetic approaches to confirm expression of these cold shock proteins (YmcF and YnfQ) at reduced temperatures and identify the noncanonical ATT start codons that initiate their translation. These proteins are conserved in related Gram-negative bacteria and are predicted to be structured, which, in combination with their cold shock upregulation, suggests that they are likely to have biological roles in the cell. These results reveal that previously unknown factors are involved in the response of E. coli to lowered temperatures and suggest that further nonannotated, stress-regulated E. coli microproteins may remain to be found. More broadly, comparative proteomics may enable discovery of regulated, and therefore potentially functional, products of smORF translation across many different organisms and conditions

Enhanced fasting glucose turnover in mice with disrupted action of TUG protein in skeletal muscle

Insulin stimulates glucose uptake in 3T3-L1 adipocytes in part by causing endoproteolytic cleavage of TUG (tether containing a ubiquitin regulatory X (UBX) domain for glucose transporter 4 (GLUT4)). Cleavage liberates intracellularly sequestered GLUT4 glucose transporters for translocation to the cell surface. To test the role of this regulation in muscle, we used mice with muscle-specific transgenic expression of a truncated TUG fragment, UBX-Cter. This fragment causes GLUT4 translocation in unstimulated 3T3-L1 adipocytes. We predicted that transgenic mice would have GLUT4 translocation in muscle during fasting. UBX-Cter expression caused depletion of PIST (PDZ domain protein interacting specifically with TC10), which transmits an insulin signal to TUG. Whereas insulin stimulated TUG proteolysis in control muscles, proteolysis was constitutive in transgenic muscles. Fasting transgenic mice had decreased plasma glucose and insulin concentrations compared with controls. Whole-body glucose turnover was increased during fasting but not during hyperinsulinemic clamp studies. In muscles with the greatest UBX-Cter expression, 2-deoxyglucose uptake during fasting was similar to that in control muscles during hyperinsulinemic clamp studies. Fasting transgenic mice had increased muscle glycogen, and GLUT4 targeting to T-tubule fractions was increased 5.7-fold. Whole-body oxygen consumption (VO2), carbon dioxide production (VCO2), and energy expenditure were increased by 12-13%. After 3 weeks on a high fat diet, the decreased fasting plasma glucose in transgenic mice compared with controls was more marked, and increased glucose turnover was not observed; the transgenic mice continued to have an increased metabolic rate. We conclude that insulin stimulates TUG proteolysis to translocate GLUT4 in muscle, that this pathway impacts systemic glucose homeostasis and energy metabolism, and that the effects of activating this pathway are maintained during high fat diet-induced insulin resistance in mice

APC7 mediates ubiquitin signaling in constitutive heterochromatin in the developing mammalian brain

Neurodevelopmental cognitive disorders provide insights into mechanisms of human brain development. Here, we report an intellectual disability syndrome caused by the loss of APC7, a core component of the E3 ubiquitin ligase anaphase promoting complex (APC). In mechanistic studies, we uncover a critical role for APC7 during the recruitment and ubiquitination of APC substrates. In proteomics analyses of the brain from mice harboring the patient-specific APC7 mutation, we identify the chromatin-associated protein Ki-67 as an APC7-dependent substrate of the APC in neurons. Conditional knockout of the APC coactivator protein Cdh1, but not Cdc20, leads to the accumulation of Ki-67 protein in neurons in&nbsp;vivo, suggesting that APC7 is required for the function of Cdh1-APC in the brain. Deregulated neuronal Ki-67 upon APC7 loss localizes predominantly to constitutive heterochromatin. Our findings define an essential function for APC7 and Cdh1-APC in neuronal heterochromatin regulation, with implications for understanding human brain development and disease