S-100 modulates Ca2+-independent phosphorylation of an endogenous protein (Mr = 19K) in brain.

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 X 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.