In Vitro Phosphorylation of Insulin Receptor Substrate 1 by Protein Kinase C-zeta: Functional Analysis and Identification of Novel Phosphorylation Sites

Biochemistry. May, 2004  |  Pubmed ID: 15134463

Protein kinase C-zeta (PKC-zeta) participates both in downstream insulin signaling and in the negative feedback control of insulin action. Here we used an in vitro approach to identify PKC-zeta phosphorylation sites within insulin receptor substrate 1 (IRS-1) and to characterize the functional implications. A recombinant IRS-1 fragment (rIRS-1(449)(-)(664)) containing major tyrosine motifs for interaction with phosphatidylinositol (PI) 3-kinase strongly associated to the p85alpha subunit of PI 3-kinase after Tyr phosphorylation by the insulin receptor. Phosphorylation of rIRS-1(449)(-)(664) by PKC-zeta induced a prominent inhibition of this process with a mixture of classical PKC isoforms being less effective. Both PKC-zeta and the classical isoforms phosphorylated rIRS-1(449)(-)(664) on Ser(612). However, modification of this residue did not reduce the affinity of p85alpha binding to pTyr-containing peptides (amino acids 605-615 of rat IRS-1), as determined by surface plasmon resonance. rIRS-1(449)(-)(664) was then phosphorylated by PKC-zeta using [(32)P]ATP and subjected to tryptic phosphopeptide mapping based on two-dimensional HPLC coupled to mass spectrometry. Ser(498) and Ser(570) were identified as novel phosphoserine sites targeted by PKC-zeta. Both sites were additionally confirmed by phosphopeptide mapping of the corresponding Ser --> Ala mutants of rIRS-1(449)(-)(664). Ser(570) was specifically targeted by PKC-zeta, as shown by immunoblotting with a phosphospecific antiserum against Ser(570) of IRS-1. Binding of p85alpha to the S570A mutant was less susceptible to inhibition by PKC-zeta, when compared to the S612A mutant. In conclusion, our in vitro data demonstrate a strong inhibitory action of PKC-zeta at the level of IRS-1/PI 3-kinase interaction involving multiple serine phosphorylation sites. Whereas Ser(612) appears not to participate in the negative control of insulin signaling, Ser(570) may at least partly contribute to this process.

Lack of Myoglobin Causes a Switch in Cardiac Substrate Selection

Circulation Research. Apr, 2005  |  Pubmed ID: 15817884

Myoglobin is an important intracellular O2 binding hemoprotein in heart and skeletal muscle. Surprisingly, disruption of myoglobin in mice (myo-/-) resulted in no obvious phenotype and normal cardiac function was suggested to be mediated by structural alterations that tend to steepen the oxygen pressure gradient from capillary to mitochondria. Here we report that lack of myoglobin causes a biochemical shift in cardiac substrate utilization from fatty acid to glucose oxidation. Proteome and gene expression analysis uncovered key enzymes of mitochondrial beta-oxidation as well as the nuclear receptor PPAR to be downregulated in myoglobin-deficient hearts. Using FDG-PET we showed a substantially increased in vivo cardiac uptake of glucose in myo-/- mice (6.7+/-2.3 versus 0.8+/-0.5% of injected dose in wild-type, n=5, P<0.001), which was associated with an upregulation of the glucose transporter GLUT4. The metabolic switch was confirmed by 13C NMR spetroscopic isotopomer studies of isolated hearts which revealed that [1,6-13C2]glucose utilization was increased in myo-/- hearts (38+/-8% versus 22+/-5% in wild-type, n=6, P<0.05), and concomitantly, [U-13C16]palmitate utilization was decreased in the myoglobin-deficient group (42+/-6% versus 63+/-11% in wild-type, n=6, P<0.05). Because of the O2-sparing effect of glucose utilization, the observed shift in substrate metabolism benefits energy homoeostasis and therefore represents a molecular adaptation process allowing to compensate for lack of the cytosolic oxygen carrier myoglobin. Furthermore, our data suggest that an altered myoglobin level itself may be a critical determinant for substrate selection in the heart. The full text of this article is available online at http://circres.ahajournals.org.

Lipopolysaccharide-induced Tyrosine Nitration and Inactivation of Hepatic Glutamine Synthetase in the Rat

Hepatology (Baltimore, Md.). May, 2005  |  Pubmed ID: 15830392

Glutamine synthetase (GS) in the liver is restricted to a small perivenous hepatocyte population and plays an important role in the scavenging of ammonia that has escaped the periportal urea-synthesizing compartment. We examined the effect of a single intraperitoneal injection of lipopolysaccharide (LPS) in vivo on glutamine synthesis in rat liver. LPS injection induced expression of inducible nitric oxide synthase, which was maximal after 6 to 12 hours but returned toward control levels within 24 hours. Twenty-four hours after LPS injection, an approximately fivefold increase in tyrosine-nitrated proteins in liver was found, and GS protein expression was decreased by approximately 20%, whereas GS activity was lowered by 40% to 50%. GS was found to be tyrosine-nitrated in response to LPS, and immunodepletion of tyrosine-nitrated proteins decreased GS protein by approximately 50% but had no effect on GS activity. Together with the finding via mass spectrometry that peroxynitrite-induced inactivation of purified GS is associated with nitration of the active site tyrosine residue, our data suggest that tyrosine nitration critically contributes to inactivation of the enzyme. In line with GS inactivation, glutamine synthesis from ammonia (0.3 mmol/L) in perfused livers from 24-hour LPS-treated rats was decreased by approximately 50%, whereas urea synthesis was not significantly affected. In conclusion, LPS impairs hepatic ammonia detoxification by both downregulation of GS and its inactivation because of tyrosine nitration. The resulting defect of perivenous scavenger cell function with regard to ammonia elimination may contribute to sepsis-induced development of hyperammonemia in patients who have cirrhosis.

LPS-induced Tyrosine Nitration of Hepatic Glutamine Synthetase

Hepatology (Baltimore, Md.). Aug, 2005  |  Pubmed ID: 16025504

The Polysaccharide Scaffold of PrP 27-30 is a Common Compound of Natural Prions and Consists of Alpha-linked Polyglucose

Biological Chemistry. Nov, 2005  |  Pubmed ID: 16307480

An inert polysaccharide scaffold identified as a 5-15% component of prion rods (PrP 27-30) is unambiguously distinguishable from the N-glycosyl groups and the GPI anchor of PrP, and consists predominantly of 1,4-linked glucose with some branching via 1,4,6-linked glucose. We show that this polysaccharide scaffold is a common secondary component of prions found in hamster full-length PrP(Sc), prion rods and in mouse ScN2a prions from cell culture. The preparation from prion rods was improved, resulting in a polysaccharide scaffold free of remaining infectivity. Furthermore, we determined the stereochemistry of the glycoside linkages as pre-dominantly if not entirely alpha-glycosidic. The origin of the polysaccharide, its interaction with PrP and its potential relation to glycogen and corpora amylacea are discussed.

Singlet Oxygen Inactivates Protein Tyrosine Phosphatase-1B by Oxidation of the Active Site Cysteine

Biological Chemistry. Oct-Nov, 2006  |  Pubmed ID: 17081112

Singlet oxygen ((1)O(2)), an electronically excited form of molecular oxygen, is a mediator of biological effects of ultraviolet A radiation, stimulating signaling cascades in human cells. We demonstrate here that (1)O(2) generated by photosensitization or by thermodecomposition of 3,3'-(1,4-naphthylidene)dipropionate-1,4-endoperoxide inactivates isolated protein tyrosine phosphatases (PTPases). PTPase activities of PTP1B or CD45 were abolished by low concentrations of (1)O(2), but were largely restored by post-treatment with dithiothreitol. Electrospray ionization mass spectrometry analysis of tryptic digests of PTP1B exposed to (1)O(2) revealed oxidation of active-site Cys215 as the only cysteine residue oxidized. In summary, (1)O(2) may activate signaling cascades by interfering with phosphotyrosine dephosphorylation.

Monitoring Conformational Changes During the Catalytic Cycle of OpuAA, the ATPase Subunit of the ABC Transporter OpuA from Bacillus Subtilis

The Biochemical Journal. Jun, 2008  |  Pubmed ID: 18321243

The ABC transporter (ATP-binding-cassette transporter) OpuA is one of five membrane transport systems in Bacillus subtilis that mediate osmoprotection by importing compatible solutes. Just like all bacterial and archaeal ABC transporters that catalyse the import of substrates, OpuA (where Opu is osmoprotectant uptake) is composed of an ATPase subunit (OpuAA), a transmembrane subunit (OpuAB) and an extracellular substrate-binding protein (OpuAC). In contrast with many well-known ABC-ATPases, OpuAA is composed not only of a catalytic and a helical domain but also of an accessory domain located at its C-terminus. The paradigm of such an architecture is MalK, the ABC-ATPase of the maltose importer of Escherichia coli, for which detailed structural and functional information is available. In the present study, we have applied solution FRET (Förster resonance energy transfer) techniques using two single cysteine mutants to obtain initial structural information on the architecture of the OpuAA dimer in solution. Analysing our results in detail and comparing them with the existing MalK structures revealed that the catalytic and helical domains adopted an arrangement similar to those of MalK, whereas profound differences in the three-dimensional orientation of the accessory domain, which contains two CBS (cystathionine beta-synthetase) domains, were observed. These results shed new light on the role of this accessory domain present in a certain subset of ABC-ATPase in the fine-tuning of three-dimensional structure and biological function.

Genetic and Biochemical Analysis of the Serine/threonine Protein Kinases PknA, PknB, PknG and PknL of Corynebacterium Glutamicum: Evidence for Non-essentiality and for Phosphorylation of OdhI and FtsZ by Multiple Kinases

Molecular Microbiology. Nov, 2009  |  Pubmed ID: 19788543

We previously showed that the 2-oxoglutarate dehydrogenase inhibitor protein OdhI of Corynebacterium glutamicum is phosphorylated by PknG at Thr14, but that also additional serine/threonine protein kinases (STPKs) can phosphorylate OdhI. To identify these, a set of three single (DeltapknA, DeltapknB, DeltapknL), five double (DeltapknAG, DeltapknAL, DeltapknBG, DeltapknBL, DeltapknLG) and two triple deletion mutants (DeltapknALG, DeltapknBLG) were constructed. The existence of these mutants shows that PknA, PknB, PknG and PknL are not essential in C. glutamicum. Analysis of the OdhI phosphorylation status in the mutant strains revealed that all four STPKs can contribute to OdhI phosphorylation, with PknG being the most important one. Only mutants in which pknG was deleted showed a strong growth inhibition on agar plates containing glutamine as carbon and nitrogen source. Thr14 and Thr15 of OdhI were shown to be phosphorylated in vivo, either individually or simultaneously, and evidence for up to two additional phosphorylation sites was obtained. Dephosphorylation of OdhI was shown to be catalysed by the phospho-Ser/Thr protein phosphatase Ppp. Besides OdhI, the cell division protein FtsZ was identified as substrate of PknA, PknB and PknL and of the phosphatase Ppp, suggesting a role of these proteins in cell division.

Characterization of O-phosphohydroxyproline in Rat {alpha}-crystallin A

The Journal of Biological Chemistry. Oct, 2010  |  Pubmed ID: 20682783

Post-translational modifications have major importance for the structure and function of a multiplicity of proteins. Phosphorylation is a widespread phenomenon among eukaryotic proteins. Whereas O-phosphorylation on the side chains of serine, threonine, and tyrosine in proteins is well known and has been studied extensively, to our knowledge the endogenous phosphorylation of hydroxyproline has not previously been reported. In the present work, we provide evidence for the first time that O-phosphohydroxyproline (Hyp(P)) is a proteinogenic amino acid. To detect Hyp(P) in proteins we generated a Hyp(P)-specific polyclonal antibody. We could identify Hyp(P) in various proteins by Western blot analysis, and we characterized the sequence position of Hyp(P) in the protein α-crystallin A by electrospray ionization-tandem mass spectrometry. Our experiments clearly demonstrate hydroxylation and subsequent phosphorylation of a proline residue in α-crystallin A in the eye as well as in heart tissue of rat.

Easy and Rapid Purification of Highly Active Nisin

International Journal of Peptides. 2011  |  Pubmed ID: 21941571

Nisin is an antimicrobial peptide produced and secreted by several L. lactis strains and is specifically active against Gram-positive bacteria. In previous studies, nisin was purified via cation exchange chromatography at low pH employing a single-step elution using 1 M NaCl. Here, we describe an optimized purification protocol using a five-step NaCl elution to remove contaminants. The obtained nisin is devoid of impurities and shows high bactericidal activity against the nisin-sensitive L. lactis strain NZ9000. Purified nisin exhibits an IC(50) of ~3 nM, which is a tenfold improvement as compared to nisin obtained via the one-step elution procedure.

Identification of Fibroblast Growth Factor 1 (FGF-1) in a Black Market Product

Drug Testing and Analysis. Nov-Dec, 2011  |  Pubmed ID: 21998075

The use of growth factors for accelerated healing of sports injuries is restricted under the terms of the World Anti-Doping Agency (WADA) anti-doping code. Cheating athletes have used the black market as a source of performance-enhancing substances. Drugs that currently undergo clinical trials are frequently offered--despite the unknown health risks associated with the administration of unapproved pharmaceuticals. Recently, a new growth factor (referred to as fibroblast growth factor 1/FGF-1) with known effects on the repair and regeneration of damaged tissue was detected in an unlabelled black market product confiscated by the German customs. The identification of the protein was achieved by one- and two-dimensional polyacrylamide gel electrophoresis (SDS-PAGE and 2D-PAGE), different proteolytic digestions, immunological methods and nano-liquid chromatography high-resolution/high-accuracy Orbitrap mass spectrometry. The SDS-PAGE analysis revealed slight differences concerning the molecular weight of recombinant human and black market FGF-1. Using in-gel proteolysis, a truncation or modification located at the N-terminus of the protein was suggested. These findings demonstrate that drug candidates without clinical approval can be readily obtained from the black market, regardless of potential dangerous consequences for the consumer, which corroborates the necessity of proactive and preventive doping control approaches. In that regard, physiological concentrations of blood and urine specimens collected from healthy individuals were analyzed and were found to range below 28 pg/ml in urine, while there was no detectable FGF-1 in plasma.