Phosphoproteins of the Adrenal Chromaffin Granule Membrane

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Abstract
A fraction of chromaffin granule membranes [prepared from bovine adrenal medulla], contained a number of substrates for endogenous protein kinase activity and endogenous phosphatase activity. The major 32P-labeled polypeptide of MW 43,000 appeared to be the .alpha.-subunit of pyruvate dehydrogenase of residual mitochondria. Several polypeptides showed cAMP stimulation of phosphorylation, of which the major polypeptide of MW 59,000 shows half-maximal phosphorylation with 0.49 .mu.M cAMP. The phosphorylation of several other polypeptides is inhibited at high cAMP concentrations. From studies with immunoprecipitation and 2-dimensional electrophoresis, .alpha.- and .beta.-tubulin and actin were absent from the granule membranes. 32P labeling of a proportion of the copies of dopamine-.beta.-hydroxylase was demonstrated. The majority of the substrates for endogenous protein kinase activity are probably on the cytoplasmic side of the granule membrane.