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Neurofilament Phosphorylation in Cultured Bovine Adrenal Chromaffin Cells Is Stimulated by Phorbol Ester

Abstract Primary cultures of bovine adrenal chromaffin cells contain neurofilament proteins that are hypophosphorylated. When the cells were grown in medium containing 32Pi and 0.1 μM 12‐O‐tetradecanoyl‐phorbol 13‐acetate (TPA), 32P‐labelling of the three neurofilament subunits was increased 6‐ to 20‐fold relative to controls, the highest level of stimulation occurring for the mid‐sized subunit. Addition of the protease inhibitor leupeptin to the growth medium had no effect on TPA‐stimulated phosphorylation. The increased 32P incorporation was accompanied by a marked reduction in the gel electrophoretic mobilities of the two largest subunits. The augmented phosphorylation was observed 10 min after addition of TPA to a concentration of 0.1 μM or after 1 h of incubation in the presence of 0.01 μM TPA. One‐dimensional peptide mapping and phosphoamino acid analysis indicated that TPA stimulated the phosphorylation of seryl residues at new sites in the mid‐sized subunit. All of the latter subunit contained in the cytoskeletal fraction of chromaffin cells was converted to a more highly phosphorylated state after the cells were grown in the presence of TPA for 1 h.