Determination of phosphorylation levels of tyrosine hydroxylase by electrospray mass spectrometry

The in vivo phosphorylation and glycosylation of human insulin-like growth factor-binding protein-5

Mass spectrometry is often used to determine post-translational modifications by analysis of tryptic digests of proteins. Here we demonstrate that the analysis of tryptic peptides together with analysis of the full-length protein provided optimal characterization of insulin-like growth factor-binding protein-5 (IGFBP-5) phosphorylation and glycosylation. IGFBP-5 binds insulin-like growth factors with high affinity and has important roles in cell survival, differentiation, and apoptosis. Until now, the primary structure of IGFBP-5 has been incompletely defined. We analyzed human IGFBP-5 from T47D cells by mass spectrometry to determine all of the in vivo post-translational modifications. In full-length IGFBP-5, 31% of the protein was unmodified, 37% was monophosphorylated, and 4% was diphosphorylated with no other modification. The remaining 27% was glycosylated, more than half of which was also monophosphorylated. The major phosphorylation site was Ser(96) in the central domain, and a minor phosphorylation site was Ser(248) near the C terminus. Neither site was phosphorylated in vitro by casein kinase 2, ruling it out as the in vivo kinase. An in vivo phosphorylation site was also found in IGFBP-2 at an analogous position, Ser(106). IGFBP-5 was heterogeneously O-glycosylated mainly by sialylated core 1 type glycans. The most abundant structure contained N-acetylhexosamine, hexose, and two N-acetylneuraminic acid carbohydrates. A small amount of sialylated core 2 type glycan was also present. Phosphorylation and O-glycosylation both affected IGFBP-5 binding to heparin but not insulin-like growth factor binding or ternary complex formation with the acid-labile subunit. The results reveal the first description of the in vivo phosphorylation of IGFBP-5 and its glycan composition.

Metal Ligand Affinity Pipettes and Bioreactive Alkaline Phosphatase Probes

An alkaline phosphatase-bioreactive probe, in which the enzyme is covalently bound to the mass spectrometry target, has been developed for studies of phosphoproteins. The bioreactive probe was used in combination with affinity capture and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry to study hydrolysis of several phosphoproteins found in human saliva. Human salivary proteins were extracted from diluted human saliva with immobilized metal-affinity pipettes, which under defined conditions bound the phosphoproteins of interest preferentially over histatins. Phosphoproteins were eluted directly from the affinity pipettes to the bioreactive probe with diluted ammonium hydroxide, which provided conditions appropriate for hydrolysis by the alkaline phosphatase covalently bound to the probe surface. Results indicate the combination of metal-affinity pipette extraction, alkaline phosphatase-bioreactive probes, and matrix-assisted laser desorption/ionization mass spectrometry is an effective way to find and characterize phosphoproteins, known and unknown, in complex mixtures. Facile hydrolysis of human salivary phosphoproteins by the bioreactive probes was readily observed.

Role of protein phosphatase 2C from bovine adrenal chromaffin cells in the dephosphorylation of phospho-serine 40 tyrosine hydroxylase

Tyrosine hydroxylase (TH) is the rate-limiting enzyme in the synthesis of catecholamines. It is dephosphorylated by protein phosphatase (PP) 2A and PP2C. In this study we used a fixed amount of bacterially expressed rat TH (5 microM), phosphorylated only at serine 40 (pSer40TH), to determine the PP activities against this site that are present in extracts from the bovine adrenal cortex, adrenal medulla, adrenal chromaffin cells and rat striatum. We found that PP2C was the main TH phosphatase activity in extracts from the adrenal medulla and adrenal chromaffin cells. In adrenal cortex extracts PP2C and PP2A activities toward pSer40TH did not differ significantly. PP2A was the main TH phosphatase activity in extracts from rat striatum. Kinetic studies with extracts from adrenal chromaffin cells showed that when higher concentrations of pSer40TH (> 5 microM) were used the activity of PP2C increased more than the activity of PP2A. PP2C was maximally activated by 1.25 mM Mn2+ and by 5 mM Mg2+ but was inhibited by calcium. Our data suggest a more important role for PP2C than was previously suggested in the dephosphorylation of serine 40 on TH.

Regulation of tyrosine hydroxylase by stress-activated protein kinases

Recombinant human tyrosine hydroxylase (hTH1) was found to be phosphorylated by mitogen and stress-activated protein kinase 1 (MSK1) at Ser40 and by p38 regulated/activated kinase (PRAK) on Ser19. Phosphorylation by MSK1 induced an increase in Vmax and a decrease in Km for 6-(R)-5,6,7,8-tetrahydrobiopterin (BH4), while these kinetic parameters were unaffected as a result of phosphorylation by PRAK. Phosphorylation of both Ser40 and Ser19 induced a high-affinity binding of 14-3-3 proteins, but only the interaction of 14-3-3 with Ser19 increased the hTH1 activity. The 14-3-3 proteins also inhibited the rate of dephosphorylation of Ser19 and Ser40 by 82 and 36%, respectively. The phosphorylation of hTH1 on Ser19 caused a threefold increase in the rate of phosphorylation of Ser40. These studies provide new insights into the possible roles of stress-activated protein kinases in the regulation of catecholamine biosynthesis.

Phosphorylation of Ser(19) alters the conformation of tyrosine hydroxylase to increase the rate of phosphorylation of Ser(40)

The effect of phosphorylation on the shape of tyrosine hydroxylase (TH) was studied directly using gel filtration and indirectly using electrospray ionization mass spectrometry. Phosphorylation of Ser(19) and Ser(40) produced a TH molecule with a more open conformation than the non-phosphorylated form. The conformational effect of Ser(19) phosphorylation is less pronounced than that of the Ser(40) phosphorylation. The effect of Ser(19) and Ser(40) phosphorylation appears to be additive. Binding of dopamine produced a more compact form when compared with the non-dopamine-bound TH. The interdependence of Ser(19) and Ser(40) phosphorylation was probed using electrospray ionization mass spectrometry. The rate constants for the phosphorylation of Ser(19) and Ser(40) were determined by electrospray ionization mass spectrometry using a consecutive reaction model. The rate constant for the phosphorylation of Ser(40) is approximately 2- to 3-fold higher if Ser(19) is already phosphorylated. These results suggest that phosphorylation of Ser(19) alters the conformation of tyrosine hydroxylase to allow increased accessibility of Ser(40) to kinases.

Protein phosphorylation degree: determination by capillary liquid chromatography and inductively coupled plasma mass spectrometry

Capillary liquid chromatography (muLC) interfaced to inductively coupled plasma mass spectrometry (ICPMS) is introduced as a new micromethod to determine the phosphorylation degree in phosphoproteins and phosphopeptides containing cysteine and/or methionine residues. The stoichiometric phosphorus to sulfur (31P to 32S) ratio is experimentally determined by muLC-ICPMS and converted into the degree of phosphorylation using protein/ peptide sequence information. The method is applied to the phosphoproteins beta-casein, beta-casein, and recombinant protein kinase A catalytic subunit and to synthetic phosphopeptides. The accurate data obtained by muLC-ICPMS allow quantitative assessment of the compound-specific discrimination of the electrospray ionization process between nonphosphorylated and phosphorylated proteins and peptides.

Current literature in mass spectrometry

In order to keep subscribers up-to-date with the latest developments in their field, John Wiley & Sons are providing a current awareness service in each issue of the journal. The bibliography contains newly published material in the field of mass spectrometry. Each bibliography is divided into 11 sections: 1 Books, Reviews & Symposia; 2 Instrumental Techniques & Methods; 3 Gas Phase Ion Chemistry; 4 Biology/Biochemistry: Amino Acids, Peptides & Proteins; Carbohydrates; Lipids; Nucleic Acids; 5 Pharmacology/Toxicology; 6 Natural Products; 7 Analysis of Organic Compounds; 8 Analysis of Inorganics/Organometallics; 9 Surface Analysis; 10 Environmental Analysis; 11 Elemental Analysis. Within each section, articles are listed in alphabetical order with respect to author (3 Weeks journals - Search completed at 28th. June 2000)