In an attempt to disclose phosphoproteome of the normal rat glomerulus, which is assumed to play critical roles in the physiology and pathophysiology of the kidney, we have conducted a proteomic study by combining 1-DE or 2-DE separation and immunochemical analyses using anti-phosphotyrosine antibodies in order to identify tyrosine-phosphorylated proteins in the normal rat glomerulus. Western blotting analysis revealed the presence of tyrosine-phosphorylated proteins of 62, 70, 120, 145 ,200, 220-kDa, of which those of 62, 70, 120, 145, 220-kDa were predominantly detected in the glomerulus, while others were commonly present in the three compartments; glomerulus, cortex, and medulla. These phosphorylation was augmented by orthovanadate, and abolished in the presence of phosphotyrosine but not of phoshoserine or phosphothreonine. Immunofluorescence microscopy indicated the predominant localization of phosphotyrosine staining along capillary walls in the glomerulus with less intense staining along the tubular epithelium. Immunoelectron microscopy also indicated the predominant localization of phosphotyrosin immunoreactivity to the basal membrane of glomerular epithelial cell (podocyte) foot processes adjacent to GBM and the basement of slit diaphragm. With MALDI-TOF MS and LC-MS/MS analysis, we have so far identified 4 tyrosine-phosphorylated proteins as talin (220-kDa), vinculin (120-kDa), ezrin (74-kDa), Hsc70 (70-kDa). Immunoblotting analysis using antibodies against talin, vinculin and Hsc70 revealed their ubiquitous expression in all the three compartments of kidney. These results suggest that talin, vinculin and ezrin (membrane cytoskeleton-associated proteins), and Hsc70 (a constitutive member of Hsp 70 family) are phosphorylated almost exclusively in the glomerulus, presumably in podocytes, highly specialized epithelial cells structurally adapted to facilitate bulk flow of the glomerular filtrate through their intercellular structures, slit diaphragms.
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