Evidence That the Synaptic Phosphoprotein B‐50 Is Localized Exclusively in Nerve Tissue

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Abstract
The localization of the phosphoprotein B-50 (MW 48,000, isoelectric point 4.5) in the rat was studied. Inspection of endogenous phosphorylation patterns of the particulate as well as the cytosolic subcellular fractions from a variety of peripheral organs failed to demonstrate phosphorylation of a MW 48,000 protein. Only in the particulate fractions from brain tissue was there endogenous phosphorylation of the B-50 protein. Two-dimensional analysis (isoelectric focusing and sodium dodecyl sulfate polyacrylamide gel electrophoresis) and an immunochemical detection method employing an anti-B-50 antiserum revealed the presence of B-50 in particulate material from brain, but not in that of other tissues. The data were interpreted as pointing to the localization of B-50 in nervous tissue. The regional distribution of endogenous B-50 phosphorylation was studied using synaptosomal plasma membranes (SPM) obtained from individual rat brain regions. The highest value was found in SPM of septal origin, the lowest in SPM from the medulla spinalis. The relationship of the high value for B-50 phosphorylation in the septum to the sensitivity of that brain area to ACTH1-24 is discussed.