Evidence that the heat-stable protein activator of phosphorylase phosphatase is histone H1

The heat-stable protein activator of phosphorylase phosphatase [FEBS Lett. 146, 331-334 (1982)] was located in the nuclear fraction. The activator was isolated from the nuclear fraction of various tissues. The largest amount was obtained from calf thymus and swine renal cortex. The activator protein from swine renal cortex was purified to apparent homogeneity. The activator had the same mobility as authentic histone H1 on sodium dodecyl sulfate or acetic acid/urea polyacrylamide gel electrophoresis. Phosphorylase phosphatase was activated by histone H1 or purified activator in an identical manner. Other histones were without effect or inhibited phosphorylase phosphatase activity. It is concluded that the heat-stable protein activator of phosphorylase phosphatase is histone H1.