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S‐100 Modulates Ca2+‐Independent Phosphorylation of an Endogenous Protein (Mr= 19K) in Brain

Abstract: A new brain enzyme (tentatively named protein kinase X), which catalyzes protamine phosphorylation modulated by S‐100, was reported recently. An endogenous substrate protein (Mr= 19K) for protein kinase × was isolated from brain by means of S‐100‐Sepharose 4B affinity chromatography. S‐100, but not calmodulin, promoted phosphorylation of the 19K Mr protein in a Ca2+‐independent manner, and this reaction was inhibited by gossypol. The substrate protein, localized in the particulate fraction, was present at a much higher level in brain from adult than neonatal rats (2‐day‐old), a developmental change similar to that seen for protein kinase X. It is suggested that a protein phosphorylation system modulated by S‐100 exists in brain, and that this process may be involved in regulation of certain neural functions.