Lysine-rich histone H1 kinase from soybean hypocotyl.

A protein kinase (ATP: histone phosphotransferase) with high specificity for the phosphorylation of the very lysine-rich histone H1 has been partially purified and characterized from soybean hypocotyl. The enzyme has a molecular weight of about 48,500. Its activity and sedimentation behavior are refractory to cyclic nucleoside monophosphates. No significant amount of cyclic AMP or cyclic GMP binding activity could be detected in the crude or partially purified enzyme preparations. Km for ATP and histone H1 are 0.4 μM and 0.7 μM, respectively. The enzyme requires Mg²⁺ or Mn²⁺ for activity, while addition of 0.5 mM Ca²⁺, Zn²⁺ or Hg²⁺ results in 50% inhibition. Arginine-rich histones H3 and H4 are inhibitory to histone H1 phosphorylation; these histones affect the Vmax of the enzyme, but not the Km for histone H1.