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Phosphoproteins of the Adrenal Chromaffin Granule Membrane

Abstract: A fraction of chromaffin granule membranes contained a number of substrates for endogenous protein kinase activity as well as endogenous phosphatase activity. The major 32P‐labelled polypeptide of molecular weight 43,000 appeared to be the α‐subunit of pyruvate dehydrogenase of residual mitochondria. Several polypeptides showed cyclic AMP stimulation of phosphorylation of which the major polypeptide of molecular weight 59,000 shows half‐maximal phosphorylation with 0.49 μM cyclic AMP. The phosphorylation of several other polypeptides is inhibited at high cyclic AMP concentrations. From studies with immunoprecipitation and two‐dimensional electrophoresis it was found that α‐ and β‐tubulin and actin were absent from the granule membranes. However 32P labelling of a proportion of the copies of dopamine‐β‐hydroxylase was demonstrated. The majority of the substrates for endogenous protein kinase activity are probably on the cytoplasmic side of the granule membrane.