Staying under the radar - the multifunctional LANA Protein

Many viruses have developed numerous strategies to recruit and take advantage of cellular protein degradation pathways to evade the cellular viral immune system. One such virus is the Kaposi´s Sarcoma associated herpesvirus (KSHV), first discovered in Kaposi´s Sarcoma lesions found in AIDS patients. Latency-Associated Nuclear Antigen (LANA) is a KSHV multifunctional protein responsible for tethering viral DNA to the chromosome ensuring maintenance and segregation of the viral genome during cell division. Besides its main role of viral maintenance, LANA also physically interacts with several host proteins to modulate cell functions. One such function is to recruit the EC5S ubiquitin-ligase complex by interacting with Elongin BC complex and Cullin 5 protein, which in turn ubiquitinate substrates such as NF-κB and p53 to allow persistent viral infection. Like any other post-translation modifications, ubiquitination is reversible through deubiquitination enzymes (DUBs). LANA also interacts with ubiquitin specific protease 7 (USP7), a deubiquitination enzyme involved in regulation of several proteins including p53. Interaction with USP7 is made through a conserved peptide motif, which is also present in LANA. This work addresses the role of LANA in the recruitment and modulation of the ubiquitination and deubiquitination pathways. Despite the continued efforts in uncovering new LANA interacting partners to form a functional EC5S ubiquitin-ligase complex, only MHV-68 LANA interacted directly with Elongin BC, other interactions were not direct and may require a linker protein. On the other hand, LANA interaction with USP7 was able to be analysed by X-ray structure determination. In addition to a conserved P/AxxS motif, a novel Glutamine (Gln) residue from KSHV LANA was shown to make a specific interaction with USP7. This Gln residue is also present in other herpesvirus protein and hence it might be a conserved motif within herpesviruses.

Arsenite interferes with protein folding and triggers formation of protein aggregates in yeast.

Several metals and metalloids profoundly affect biological systems, but their impact on the proteome and mechanisms of toxicity are not fully understood. Here, we demonstrate that arsenite causes protein aggregation in Saccharomyces cerevisiae. Various molecular chaperones were found to be associated with arsenite-induced aggregates indicating that this metalloid promotes protein misfolding. Using in vivo and in vitro assays, we show that proteins in the process of synthesis/folding are particularly sensitive to arsenite-induced aggregation, that arsenite interferes with protein folding by acting on unfolded polypeptides, and that arsenite directly inhibits chaperone activity. Thus, folding inhibition contributes to arsenite toxicity in two ways: by aggregate formation and by chaperone inhibition. Importantly, arsenite-induced protein aggregates can act as seeds committing other, labile proteins to misfold and aggregate. Our findings describe a novel mechanism of toxicity that may explain the suggested role of this metalloid in the etiology and pathogenesis of protein folding disorders associated with arsenic poisoning.

Tumor necrosis factor-alpha mediates changes in tissue protein turnover in a rat cancer cachexia model.

Rats bearing the Yoshida AH-130 ascites hepatoma showed enhanced fractional rates of protein degradation in gastrocnemius muscle, heart, and liver, while fractional synthesis rates were similar to those in non-tumor bearing rats. This hypercatabolic pattern was associated with marked perturbations of the hormonal homeostasis and presence of tumor necrosis factor in the circulation. The daily administration of a goat anti-murine TNF IgG to tumor-bearing rats decreased protein degradation rates in skeletal muscle, heart, and liver as compared with tumor-bearing rats receiving a nonimmune goat IgG. The anti-TNF treatment was also effective in attenuating early perturbations in insulin and corticosterone homeostasis. Although these results suggest that tumor necrosis factor plays a significant role in mediating the changes in protein turnover and hormone levels elicited by tumor growth, the inability of such treatment to prevent a reduction in body weight implies that other mediators or tumor-related events were also involved.

Tumor necrosis factor-alpha mediates changes in tissue protein turnover in a rat cancer cachexia model.

Rats bearing the Yoshida AH-130 ascites hepatoma showed enhanced fractional rates of protein degradation in gastrocnemius muscle, heart, and liver, while fractional synthesis rates were similar to those in non-tumor bearing rats. This hypercatabolic pattern was associated with marked perturbations of the hormonal homeostasis and presence of tumor necrosis factor in the circulation. The daily administration of a goat anti-murine TNF IgG to tumor-bearing rats decreased protein degradation rates in skeletal muscle, heart, and liver as compared with tumor-bearing rats receiving a nonimmune goat IgG. The anti-TNF treatment was also effective in attenuating early perturbations in insulin and corticosterone homeostasis. Although these results suggest that tumor necrosis factor plays a significant role in mediating the changes in protein turnover and hormone levels elicited by tumor growth, the inability of such treatment to prevent a reduction in body weight implies that other mediators or tumor-related events were also involved.

Grape epicatechin conjugates prevent erythrocyte membrane protein oxidation

Epicatechin conjugates obtained from grape have shown antioxidant activity in various systems. However, how these conjugates exert their antioxidant benefits has not been widely studied. We assessed the activity of epicatechin and epicatechin conjugates on the erythrocyte membrane in the presence and absence of a peroxyl radical initiator, to increase our understanding of their mechanisms. Thus, we studied cell membrane fluidity by fluorescence anisotropy measurements, morphology of erythrocytes by scanning electron microscopy, and finally, red cell membrane proteins by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Our data showed that incubation of red cells in the presence of epicatechin derivatives altered membrane fluidity and erythrocyte morphology but not the membrane protein pattern. The presence in the medium of the peroxyl radical initiator 2,2′-azobis(amidinopropane) dihydrochloride (AAPH) resulted in membrane disruptions at all levels analyzed, causing changes in membrane fluidity, cell morphology, and protein degradation. The presence of antioxidants avoided protein oxidation, indicating that the interaction of epicatechin conjugates with the lipid bilayer might reduce the accessibility of AAPH to membranes, which could explain in part the inhibitory ability of these compounds against hemolysis induced by peroxidative insult.

Hsp90 inhibition induces both protein-specific and global changes in the ubiquitinome.

Inhibition of the essential chaperone Hsp90 with drugs causes a global perturbation of protein folding and the depletion of direct substrates of Hsp90, also called clients. Ubiquitination and proteasomal degradation play a key role in cellular stress responses, but the impact of Hsp90 inhibition on the ubiquitinome has not been characterized on a global scale. We used stable isotope labeling and antibody-based peptide enrichment to quantify more than 1500 protein sites modified with a Gly-Gly motif, the remnant of ubiquitination, in human T-cells treated with an Hsp90 inhibitor. We observed rapid changes in GlyGly-modification sites, with strong increases for some Hsp90 clients but also decreases for a majority of cellular proteins. A comparison with changes in total protein levels and protein synthesis and decay rates from a previous study revealed a complex picture with different regulatory patterns observed for different protein families. Overall the data support the notion that for Hsp90 clients GlyGly-modification correlates with targeting by the ubiquitin-proteasome system and decay, while for other proteins levels of GlyGly-modification appear to be mainly influenced by their synthesis rates. Therefore a correct interpretation of changes in ubiquitination requires knowledge of multiple parameters. Data are available via ProteomeXchange with identifier PXD001549. BIOLOGICAL SIGNIFICANCE: Proteostasis, i.e. the capacity of the cell to maintain proper synthesis and maturation of proteins, is a fundamental biological process and its perturbations have far-reaching medical implications e.g. in cancer or neurodegenerative diseases. Hsp90 is an essential chaperone responsible for the correct maturation and stability of a number of key proteins. Inhibition of Hsp90 triggers a global stress response caused by accumulation of misfolded chains, which have to be either refolded or eliminated by protein degradation pathways such as the Ubiquitin-Proteasome System (UPS). We present the first global assessment of the changes in the ubiquitinome, the subset of ubiquitin-modified proteins, following Hsp90 inhibition in human T-cells. The results provide clues on how cells respond to a specific proteostasis challenge. Furthermore, our data also suggest that basal ubiquitination levels for most proteins are influenced by synthesis rates. This has broad significance as it implies that a proper interpretation of data on ubiquitination levels necessitates simultaneous knowledge of other parameters.

Clic4, a novel protein that sensitizes β-cells to apoptosis.

OBJECTIVES: Chloride intracellular channel protein 4 (Clic4) is a ubiquitously expressed protein involved in multiple cellular processes including cell-cycle control, cell differentiation, and apoptosis. Here, we investigated the role of Clic4 in pancreatic β-cell apoptosis. METHODS: We used βTC-tet cells and islets from β-cell specific Clic4 knockout mice (βClic4KO) and assessed cytokine-induced apoptosis, Bcl2 family protein expression and stability, and identified Clic4-interacting proteins by co-immunoprecipitation and mass spectrometry analysis. RESULTS: We show that cytokines increased Clic4 expression in βTC-tet cells and in mouse islets and siRNA-mediated silencing of Clic4 expression in βTC-tet cells or its genetic inactivation in islets β-cells, reduced cytokine-induced apoptosis. This was associated with increased expression of Bcl-2 and increased expression and phosphorylation of Bad. Measurement of Bcl-2 and Bad half-lives in βTC-tet cells showed that Clic4 silencing increased the stability of these proteins. In primary islets β-cells, absence of Clic4 expression increased Bcl-2 and Bcl-xL expression as well as expression and phosphorylation of Bad. Mass-spectrometry analysis of proteins co-immunoprecipitated with Clic4 from βTC-tet cells showed no association of Clic4 with Bcl-2 family proteins. However, Clic4 co-purified with proteins from the proteasome suggesting a possible role for Clic4 in regulating protein degradation. CONCLUSIONS: Collectively, our data show that Clic4 is a cytokine-induced gene that sensitizes β-cells to apoptosis by reducing the steady state levels of Bcl-2, Bad and phosphorylated Bad.

The TRAF-interacting protein (TRAIP) is a novel E2F target with peak expression in mitosis.

The TRAF-interacting protein (TRAIP) is an E3 ubiquitin ligase required for cell proliferation. TRAIP mRNA is downregulated in human keratinocytes after inhibition of the PI3K/AKT/mTOR signaling. Since E2F transcription factors are downstream of PI3K/AKT/mTOR we investigated whether they regulate TRAIP expression. E2F1 expression significantly increased the TRAIP mRNA level in HeLa cells. Reporter assays with the 1400bp 5'-upstream promoter in HeLa cells and human keratinocytes showed that E2F1-, E2F2- and E2F4-induced upregulation of TRAIP expression is mediated by 168bp upstream of the translation start site. Mutating the E2F binding site within this fragment reduced the E2F1- and E2F2-dependent promoter activities and protein-DNA complex formation in gel shift assays. Abundance of TRAIP mRNA and protein was regulated by the cell cycle with a peak in G2/M. Expression of GFP and TRAIP-GFP demonstrated that TRAIP-GFP protein has a lower steady-state concentration than GFP despite similar mRNA levels. Cycloheximide inhibition experiments indicated that the TRAIP protein has a half-life of around four hours. Therefore, the combination of cell cycle-dependent transcription of the TRAIP gene by E2F and rapid protein degradation leads to cell cycle-dependent expression with a maximum in G2/M. These findings suggest that TRAIP has important functions in mitosis and tumorigenesis.

Regulation of gap junctions by protein phosphorylation

Gap junctions are constituted by intercellular channels and provide a pathway for transfer of ions and small molecules between adjacent cells of most tissues. The degree of intercellular coupling mediated by gap junctions depends on the number of gap junction channels and their activity may be a function of the state of phosphorylation of connexins, the structural subunit of gap junction channels. Protein phosphorylation has been proposed to control intercellular gap junctional communication at several steps from gene expression to protein degradation, including translational and post-translational modification of connexins (i.e., phosphorylation of the assembled channel acting as a gating mechanism) and assembly into and removal from the plasma membrane. Several connexins contain sites for phosphorylation for more than one protein kinase. These consensus sites vary between connexins and have been preferentially identified in the C-terminus. Changes in intercellular communication mediated by protein phosphorylation are believed to control various physiological tissue and cell functions as well as to be altered under pathological conditions.

Protein turnover, amino acid requirements and recommendations for athletes and active populations

Skeletal muscle is the major deposit of protein molecules. As for any cell or tissue, total muscle protein reflects a dynamic turnover between net protein synthesis and degradation. Noninvasive and invasive techniques have been applied to determine amino acid catabolism and muscle protein building at rest, during exercise and during the recovery period after a single experiment or training sessions. Stable isotopic tracers (13C-lysine, 15N-glycine, ²H5-phenylalanine) and arteriovenous differences have been used in studies of skeletal muscle and collagen tissues under resting and exercise conditions. There are different fractional synthesis rates in skeletal muscle and tendon tissues, but there is no major difference between collagen and myofibrillar protein synthesis. Strenuous exercise provokes increased proteolysis and decreased protein synthesis, the opposite occurring during the recovery period. Individuals who exercise respond differently when resistance and endurance types of contractions are compared. Endurance exercise induces a greater oxidative capacity (enzymes) compared to resistance exercise, which induces fiber hypertrophy (myofibrils). Nitrogen balance (difference between protein intake and protein degradation) for athletes is usually balanced when the intake of protein reaches 1.2 g·kg-1·day-1 compared to 0.8 g·kg-1·day-1 in resting individuals. Muscular activities promote a cascade of signals leading to the stimulation of eukaryotic initiation of myofibrillar protein synthesis. As suggested in several publications, a bolus of 15-20 g protein (from skimmed milk or whey proteins) and carbohydrate (± 30 g maltodextrine) drinks is needed immediately after stopping exercise to stimulate muscle protein and tendon collagen turnover within 1 h.

Regulation of Protein Turnover during Hyper-osmotic Stress in Skeletal Muscle

The purpose of this study was to examine the effect of hyper-osmotic stress on protein turnover in skeletal muscle tissue using an established in-vitro model. Rat EDL muscles were incubated in either hyper-osmotic (400 ± 10 Osm) or isoosmotic (290 ± 10 Osm) custom-modified media (Gibco). L-[14C]-U-phenylalanine (n=8) and cycloheximide (n=8) were used to quantify protein synthesis and degradation, respectively. Western blotting analyses was performed to determine the activation of protein synthesis and degradation pathways. During hyperosmotic stress, protein degradation increased (p<0.05), while protein synthesis was decreased (p<0.05) as compared to the iso-osmotic condition. The decline in protein synthesis was accompanied by a decrease (p<0.05) in p70s6 kinase phosphorylation, while the increase in protein degradation was associated with an increase (p<0.05) in autolyzed calpain. Therefore, hyper-osmotic extracellular stress results in an intracellular catabolic environment in mammalian skeletal muscle tissue.

Correlation of Protein Homeostasis With Maximum Lifespans in Endothermic Vertebrates

Cellular stress resistance has been shown to be highly correlated with longevity. However, the mechanisms conferring this stress resistance have yet to be identified. Maintenance of protein homeostasis is a critical component of cellular maintenance and stress resistance. Superior protein homeostasis capacities may thus underlie the greater stress resistance observed in longer-lived animals; however, little vertebrate data have been provided supporting this idea. I used two different experimental approaches to test the associations of protein homeostasis capacities with stress resistance and lifespan: 1) a comparison between a large set of vertebrate species with varying body masses and lifespans and 2) a comparison of long-lived Snell dwarf mice and their normal littermates. Protein homeostasis mechanisms including protein degradation activity, protein repair activity and molecular chaperone levels were examined. These measurements were performed in liver, heart and brain tissues, and isolated myoblasts. My results indicated that neither protein degradation nor protein repair were upregulated in association with enhanced stress resistance and longevity in an inter-species and intraspecies context. Furthermore, my results did show that there is a positive correlation between molecular chaperone levels and maximum lifespan (MLSP). However, there was no elevation of chaperone levels in the long-lived Snell dwarf mouse, indicating there are other mechanisms linked to their increased lifespan. Therefore, these results suggest that molecular chaperones are involved in increasing animal lifespan in an interspecies context.

INFLUENCE OF INCREASED EXTRACELLULAR LEUCINE ON THE PROTEIN METABOLIC RESPONSES DURING OSMOTIC STRESS IN SKELETAL MUSCLE

The purpose of this study was to examine the effects of increased extracellular leucine concentration on protein metabolism in skeletal muscle cells when exposed to 3 different osmotic stresses. L6 skeletal muscle cells were incubated in either a normal or supplemental leucine (1.5mM) medium set to hypo-osmotic (230 ± 10 Osm), iso-osmotic (330 ± 10 Osm) or hyper-osmotic (440 ± 10 Osm) conditions. 3H-tyrosine was used to quantify protein synthesis. Western blotting analysis was performed to determine the activation of mTOR, p70S6k, ubiquitin, actin, and μ-calpain. Hypo-osmotic stress resulted in the greatest increase in protein synthesis rate under the normal-leucine condition while iso-osmotic stress has the greatest increase under the elevated-leucine condition. Elevated-leucine condition had a decreased rate in protein degradation over the normal condition within the ubiquitin proteasome pathway (p<0.05). Leucine and hypo-osmotic stress therefore creates a favourable environment for anabolic events to occur.

CCP1/Nna1 functions in protein turnover in mouse brain: Implications for cell death in Purkinje cell degeneration mice

Purkinje cell degeneration (pcd) mice have a mutation within the gene encoding cytosolic carboxypeptidase 1 (CCP1/Nna1), which has homology to metallocarboxypeptidases. To assess the function of CCP1/Nna1, quantitative proteomics and peptidomics approaches were used to compare proteins and peptides in mutant and wild-type mice. Hundreds of peptides derived from cytosolic and mitochondrial proteins are greatly elevated in pcd mouse hypothalamus, amygdala, cortex, prefrontal cortex, and striatum. However, the major proteins detected on 2-D gel electrophoresis were present in mutant and wild-type mouse cortex and hypothalamus at comparable levels, and proteasome activity is normal in these brain regions of pcd mice, suggesting that the increase in cellular peptide levels in the pcd mice is due to reduced degradation of the peptides downstream of the proteasome. Both nondegenerating and degenerating regions of pcd mouse brain, but not wild-type mouse brain, show elevated autophagy, which can be triggered by a decrease in amino acid levels. Taken together with previous studies on CCP1/Nna1, these data suggest that CCP1/Nna1 plays a role in protein turnover by cleaving proteasome-generated peptides into amino acids and that decreased peptide turnover in the pcd mice leads to cell death.-Berezniuk, I., Sironi, J., Callaway, M. B., Castro, L. M., Hirata, I. Y., Ferro, E. S., Fricker, L. D. CCP1/Nna1 functions in protein turnover in mouse brain: Implications for cell death in Purkinje cell degeneration mice. FASEB J. 24, 1813-1823 (2010). www.fasebj.org

The 20S proteasome alpha(5) subunit of Arabidopsis thaliana carries an RNase activity and interacts in planta with the Lettuce mosaic potyvirus HcPro protein

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