Possible implication of the Golgi apparatus casein kinase in the phosphorylation of vesicle docking protein p115 Ser-940: a study with peptide substrates

CK1δ kinase activity is modulated by Chk1-mediated phosphorylation

CK1δ, a member of the casein kinase 1 family, is involved in the regulation of various cellular processes and has been associated with the pathophysiology of neurodegenerative diseases and cancer. Therefore recently, interest in generating highly specific inhibitors for personalized therapy has increased enormously. However, the efficacy of newly developed inhibitors is affected by the phosphorylation state of CK1δ. Cellular kinases phosphorylating CK1δ within its C-terminal domain have been identified but still more information regarding the role of site-specific phosphorylation in modulating the activity of CK1δ is required. Here we show that Chk1 phosphorylates rat CK1δ at serine residues 328, 331, 370, and threonine residue 397 as well as the human CK1δ transcription variants 1 and 2. CK1δ mutant proteins bearing one, two or three mutations at these identified phosphorylation sites exhibited significant differences in their kinetic properties compared to wild-type CK1δ. Additionally, CK1δ co-precipitates with Chk1 from HT1080 cell extracts and activation of cellular Chk1 resulted in a significant decrease in cellular CK1δ kinase activity. Taken together, these data point towards a possible regulatory relationship between Chk1 and CK1δ.

Extracellular phosphorylation and phosphorylated proteins: not just curiosities but physiologically important

Mining of the literature and high-throughput mass spectrometry data from both healthy and diseased tissues and from body fluids reveals evidence that various extracellular proteins can exist in phosphorylated states. Extracellular kinases and phosphatases (ectokinases and ectophosphatases) are active in extracellular spaces during times of sufficiently high concentrations of adenosine triphosphate. There is evidence for a role of extracellular phosphorylation in various physiological functions, including blood coagulation, immune cell activation, and the formation of neuronal networks. Ectokinase activity is increased in some diseases, including cancer, Alzheimer's disease, and some microbial infections. We summarize the literature supporting the physiological and pathological roles of extracellularly localized protein kinases, protein phosphatases, and phosphorylated proteins and provide an analysis of the available mass spectrometry data to annotate potential extracellular phosphorylated proteins.

Facts and artifacts in proteomics of body fluids. What proteomics of saliva is telling us?

This review briefly depicts several salient points of the current status of knowledge on salivary peptidoma. It outlines the intrinsic difficulties in its characterization connected to different factors of variability, such as: i) the high genetic polymorphisms, complicated by individual insertions/deletions and alternative splicing; ii) complex post-translational maturations comprehending different proteolytic cleavages, glycosylation, phosphorylation and sulfation processes; iii) physiological variations and different contributions to the whole. Moreover, several technological and analytical problems and pitfalls that had to be surmounted during our studies focussed on the extensive qualitative and quantitative characterization of salivary peptidoma and mainly based on LC-MS analyses of intact naturally occurring peptides are here described. The hope is that the information provided might be helpful to other groups engaged on the analysis of saliva or other body fluids for clinical applications.

p115 Interacts with the GLUT4 vesicle protein, IRAP, and plays a critical role in insulin-stimulated GLUT4 translocation

Insulin-regulated aminopeptidase (IRAP) is an abundant cargo protein of Glut4 storage vesicles (GSVs) that traffics to and from the plasma membrane in response to insulin. We used the amino terminus cytoplasmic domain of IRAP, residues 1-109, as an affinity reagent to identify cytosolic proteins that might be involved in GSV trafficking. In this way, we identified p115, a peripheral membrane protein known to be involved in membrane trafficking. In murine adipocytes, we determined that p115 was localized to the perinuclear region by immunofluorescence and throughout the cell by fractionation. By immunofluorescence, p115 partially colocalizes with GLUT4 and IRAP in the perinuclear region of cultured fat cells. The amino terminus of p115 binds to IRAP and overexpression of a N-terminal construct results in its colocalization with GLUT4 throughout the cell. Insulin-stimulated GLUT4 translocation is completely inhibited under these conditions. Overexpression of p115 C-terminus has no significant effect on GLUT4 distribution and translocation. Finally, expression of the p115 N-terminus construct has no effect on the distribution and trafficking of GLUT1. These data suggest that p115 has an important and specific role in insulin-stimulated Glut4 translocation, probably by way of tethering insulin-sensitive Glut4 vesicles at an as yet unknown intracellular site.

The Marburg virus surface protein GP is phosphorylated at its ectodomain

Marburg virus, a filovirus, contains only one transmembrane protein (GP) which is responsible for receptor recognition on target cells. GP, a type I membrane protein of approximately 220 kDa, is acylated and highly glycosylated carrying N- and O-linked sugar side chains. GP is transported through the exocytotic pathway toward the plasma membrane where budding of virions takes place. In the trans-Golgi network, GP is proteolytically activated by the prohormone convertase furin into two subunits GP(1) and GP(2). In the present paper, we provide evidence that GP undergoes an additional posttranslational modification; it is phosphorylated at its ectodomain. Phosphorylation takes place at serine residues between amino acid 260 and 273. The respective serines are located in conserved recognition sites for luminal protein kinases (protein kinase CK II and Golgi casein kinase). Consistent with this data, it was found that GP was phosphorylated in the Golgi apparatus of the expressing HeLa cells before cleavage of the molecule. GP is the first example of a viral glycoprotein with a phosphorylated ectodomain.