Tandem phosphorylation of Ser-911 and Thr-912 at the C terminus of yeast plasma membrane H+-ATPase leads to glucose-dependent activation

In recent years there has been growing interest in the post-translational regulation of P-type ATPases by protein kinase-mediated phosphorylation. Pma1 H(+)-ATPase, which is responsible for H(+)-dependent nutrient uptake in yeast (Saccharomyces cerevisiae), is one such example, displaying a rapid 5-10-fold increase in activity when carbon-starved cells are exposed to glucose. Activation has been linked to Ser/Thr phosphorylation in the C-terminal tail of the ATPase, but the specific phosphorylation sites have not previously been mapped. The present study has used nanoflow high pressure liquid chromatography coupled with electrospray electron transfer dissociation tandem mass spectrometry to identify Ser-911 and Thr-912 as two major phosphorylation sites that are clearly related to glucose activation. In carbon-starved cells with low Pma1 activity, peptide 896-918, which was derived from the C terminus upon Lys-C proteolysis, was found to be singly phosphorylated at Thr-912, whereas in glucose-metabolizing cells with high ATPase activity, the same peptide was doubly phosphorylated at Ser-911 and Thr-912. Reciprocal (14)N/(15)N metabolic labeling of cells was used to measure the relative phosphorylation levels at the two sites. The addition of glucose to carbon-starved cells led to a 3-fold reduction in the singly phosphorylated form and an 11-fold increase in the doubly phosphorylated form. These results point to a mechanism in which the stepwise phosphorylation of two tandemly positioned residues near the C terminus mediates glucose-dependent activation of the H(+)-ATPase.

Probing the dynamics of O-GlcNAc glycosylation in the brain using quantitative proteomics

The addition of the monosaccharide beta-N-acetyl-D-glucosamine to proteins (O-GlcNAc glycosylation) is an intracellular, post-translational modification that shares features with phosphorylation. Understanding the cellular mechanisms and signaling pathways that regulate O-GlcNAc glycosylation has been challenging because of the difficulty of detecting and quantifying the modification. Here, we describe a new strategy for monitoring the dynamics of O-GlcNAc glycosylation using quantitative mass spectrometry-based proteomics. Our method, which we have termed quantitative isotopic and chemoenzymatic tagging (QUIC-Tag), combines selective, chemoenzymatic tagging of O-GlcNAc proteins with an efficient isotopic labeling strategy. Using the method, we detect changes in O-GlcNAc glycosylation on several proteins involved in the regulation of transcription and mRNA translocation. We also provide the first evidence that O-GlcNAc glycosylation is dynamically modulated by excitatory stimulation of the brain in vivo. Finally, we use electron-transfer dissociation mass spectrometry to identify exact sites of O-GlcNAc modification. Together, our studies suggest that O-GlcNAc glycosylation occurs reversibly in neurons and, akin to phosphorylation, may have important roles in mediating the communication between neurons.

Supplemental activation method for high-efficiency electron-transfer dissociation of doubly protonated peptide precursors

Electron-transfer dissociation (ETD) delivers the unique attributes of electron capture dissociation to mass spectrometers that utilize radio frequency trapping-type devices (e.g., quadrupole ion traps). The method has generated significant interest because of its compatibility with chromatography and its ability to: (1) preserve traditionally labile post-translational modifications (PTMs) and (2) randomly cleave the backbone bonds of highly charged peptide and protein precursor ions. ETD, however, has shown limited applicability to doubly protonated peptide precursors, [M + 2H]2+, the charge and type of peptide most frequently encountered in "bottom-up" proteomics. Here we describe a supplemental collisional activation (CAD) method that targets the nondissociated (intact) electron-transfer (ET) product species ([M + 2H]+*) to improve ETD efficiency for doubly protonated peptides (ETcaD). A systematic study of supplementary activation conditions revealed that low-energy CAD of the ET product population leads to the near-exclusive generation of c- and z-type fragment ions with relatively high efficiency (77 +/- 8%). Compared to those formed directly via ETD, the fragment ions were found to comprise increased relative amounts of the odd-electron c-type ions (c+*) and the even-electron z-type ions (z+). A large-scale analysis of 755 doubly charged tryptic peptides was conducted to compare the method (ETcaD) to ion trap CAD and ETD. ETcaD produced a median sequence coverage of 89%-a significant improvement over ETD (63%) and ion trap CAD (77%).