No abstract

Protein misfolding is monitored by a variety of cellular "quality control" systems. Endoplasmic reticulum (ER) quality control handles misfolded secretory and membrane proteins and is well characterized. However, less is known about the quality control of misfolded cytosolic proteins (CytoQC). To study CytoQC, we have employed a genetic system in Saccharomyces cerevisiae using a transplantable degron, CL1 (1). Attachment of CL1 to the cytosolic protein Ura3p destabilizes Ura3p, targeting it for rapid proteasomal degradation. We have performed a comprehensive analysis of Ura3p-CL1 degradation requirements. As shown previously, we observe that the ER-localized ubiquitin E2 (Ubc6p, Ubc7p, and Cue1p) and E3 (Doa10p) machinery involved in ER-associated degradation (ERAD) are also responsible for the degradation of the cytosolic substrate Ura3p-CL1. Importantly, we find that the cytosol/ER membrane-localized chaperones Ydj1p and Ssa1p, known to be necessary for the ERAD of membrane proteins with misfolded cytosolic domains, are also required for the ubiquitination and degradation of Ura3p-CL1. In addition, we show a role for the Cdc48p-Npl4p-Ufd1p complex in the degradation of Ura3p-CL1. When ubiquitination is blocked, a portion of Ura3p-CL1 is ER membrane-localized. Furthermore, access to the cytosolic face of the ER is required for the degradation of CL1 degroncontaining proteins. The ER is distributed throughout the cytosol, and our data, together with previous studies, suggest that the cytosolic face of the ER membrane serves as a "platform" for the degradation of Ura3p-CL1, which may also be the case for other CytoQC substrates.Mutation, errors in transcription or translation, and cellular stress can cause alterations in amino acids that may prevent proteins from attaining their properly folded, native conformations. Protein "quality control" is an essential process monitoring protein folding, ultimately targeting misfolded proteins for degradation via the ubiquitin-proteasome system. The importance of protein quality control is best exemplified by the numerous human diseases that can result from protein misfolding due to mutational or physiological causes and include cystic fibrosis, Parkinson disease, and ␣ 1 -antitrypsin deficiency (2, 3).Distinct protein quality control systems appear to exist in various cellular compartments, including the nucleus, mitochondria, and endoplasmic reticulum (ER), 2 with the bestcharacterized system being ER quality control (4 -7). Studies of ER quality control and, in particular, ER-associated degradation (ERAD) have revealed discrete chaperone and ubiquitination machinery required for the recognition and ubiquitination of different classes of misfolded secretory or membrane proteins. Much of this work has been greatly aided by the use of "model" ER quality control substrates, such as CPY* or Ste6p*, in the yeast Saccharomyces cerevisiae (8 -12). For example, it has become clear that model membrane proteins with misfolded cytosolic domains (called ERAD-C substrates), such as St...

Protein acetylation on Lys residues is recognized as a significant post-translational modification in cells, but it is often difficult to discern the direct structural and functional effects of individual acetylation events. Here we describe a new tool, methylthiocarbonyl-aziridine, to install acetylLys mimics site-specifically into peptides and proteins by alkylation of Cys residues. We demonstrate that the resultant thiocarbamate modification can be recognized by the Brdt bromodomain and site-specific anti-acetyl-Lys antibodies, is resistant to histone deacetylase cleavage, and can confer activation of the histone acetyltransferase Rtt109 by simulating autoacetylation. We also use this approach to obtain functional evidence that acetylation of CK2 protein kinase on Lys102 can stimulate its catalytic activity.Reversible acetylation of histones catalyzed by histone acetyltransferases (HATs) and histone deacetylases (HDACs) is recognized as a central mechanism of chromatin regulation. Beyond histones, Lys acetylation has been observed in more than 2500 mammalian proteins on more than 4000 sites and has been shown to govern many biological processes.1 Several approaches are available to explore the structural and functional consequences of protein acetylation. Traditional site-directed mutagenesis is used to replace Lys with Arg or Gln to provide indirect insights into Lys acetylation. Expressed protein ligation and unnatural amino acid mutagenesis via nonsense suppression can install acetylLys (AcK) at desired protein sites but have various technical limitations.2 pcole@jhmi.edu. Supporting Information Available. Detailed experimental procedures and Figures S1-S10. This material is available free of charge via the Internet at http://pubs.acs.org. NIH Public AccessAuthor Manuscript J Am Chem Soc. Author manuscript; available in PMC 2011 July 28. NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author ManuscriptAs an analog of methyl-Lys, methyl-thiaLys can be introduced at targeted protein locations via a relatively simple strategy involving Cys alkylation with N-methyl-aminoethylbromide derivatives.3 To create an acetyl-thiaLys analog (Figure 1a), we attempted to use the corresponding acetamide reagent with Cys-containing peptide; however, no significant conversion was observed. We hypothesized that the reduced ability of the amide compounds to form three-membered ring intermediates and/or intramolecular imidate formation led to diminished activity of N-acetyl-aminoethylbromide. We then explored the reaction of Nacetyl-aziridine and a Cys-containing peptide (Figure 1b). The predominant product observed by mass spectrometry was M+42 suggesting that acetyl-aziridine treatment led to an undesired acetyl transfer by nucleophilic attack at the carbonyl rather than the aziridine methylene ( Figure S1).To reduce reactivity at the carbonyl, we explored methylthiocarbonyl-aziridine (MTCA) (Figure 1b).4 Cys alkylation with this reagent was proposed to provide methylthiocarbonylthiaLys (MTCTK), a thiocarbama...

Mitochondrial dysfunction is at the core of many diseases, ranging from inherited metabolic diseases to common conditions that are associated with ageing. While associations between ageing and mitochondrial function have been identified using mammalian models, much of the mechanistic insight has emerged from C. elegans. Mitochondrial respiration is recognized as an indicator of mitochondrial health. Seahorse XF96 respirometers are the state-of-the-art platform to assess respiration in cells, and we adapted the technique for applications involving C. elegans. Here, we provide a detailed protocol to optimise and measure respiration in C. elegans with the XF96 respirometer, including the interpretation of parameters and results. The protocol takes ~2 days to complete, excluding time spent culturing C. elegans, and includes (i) the preparation of C. elegans samples, (ii) selection and loading of compounds to be injected, (iii) preparing and executing a run with the XF96 respirometer, and (iv) post-experimental data-analysis, including normalization. In addition, we compare our XF96 application with other existing techniques, including the 8-well Seahorse XFp. The main benefits of the XF96 include the limited number of worms required and high-throughput capacity due to 96-well format.

Epigenetic events, including covalent post-translational modifications of histones, have been demonstrated to play critical roles in tumor development and progression. The transcriptional coactivator p300/CBP possesses both histone acetyltransferase (HAT) activity as well as scaffolding properties that directly influence the transcriptional activation of targeted genes. We have used a potent and specific inhibitor of p300/CBP HAT activity, C646, in order to evaluate the functional contributions of p300/CBP HAT to tumor development and progression. Here we report that C646 inhibits the growth of human melanoma and other tumor cells and promotes cellular senescence. Global assessment of the p300 HAT transcriptome in human melanoma identified functional roles in promoting cell cycle progression, chromatin assembly and activation of DNA repair pathways through direct transcriptional regulatory mechanisms. Additionally, C646 is shown to promote sensitivity to DNA damaging agents, leading to the enhanced apoptosis of melanoma cells following combination treatment with cisplatin. Together, our data suggest that p300 HAT activity mediates critical growth regulatory pathways in tumor cells and may serve as a potential therapeutic target for melanoma and other malignancies by promoting cellular responses to DNA damaging agents that are currently ineffective against specific cancers.

No abstract

MAML1 is a transcriptional coregulator originally identified as a Notch coactivator. MAML1 is also reported to interact with other coregulator proteins, such as CDK8 and p300, to modulate the activity of Notch. We, and others, previously showed that MAML1 recruits p300 to Notch-regulated genes through direct interactions with the DNA–CSL–Notch complex and p300. MAML1 interacts with the C/H3 domain of p300, and the p300–MAML1 complex specifically acetylates lysines of histone H3 and H4 tails in chromatin in vitro. In this report, we show that MAML1 potentiates p300 autoacetylation and p300 transcriptional activation. MAML1 directly enhances p300 HAT activity, and this coincides with the translocation of MAML1, p300 and acetylated histones to nuclear bodies.

Histone acetyltransferase enzymes (HATs) are important therapeutic targets, but there are few cell-based assays available for evaluating the pharmacodynamics of HAT inhibitors. Here we present the application of a FRET-based reporter, Histac, in live cell studies of p300/CBP HAT inhibition, by both genetic and pharmacologic disruption. shRNA knockdown of p300/CBP led to increased Histac FRET, suggesting a role for p300/CBP in the acetylation of the histone H4 tail present. Additionally, we describe a new p300/CBP HAT inhibitor, C107, and show that it can also increase cellular Histac FRET. Taken together, these studies provide a live cell strategy for identifying and evaluating p300/CBP inhibitors.