ABSTRACT
This chapter describes the nature and possible physiological roles of the major substrates for nuclear protein kinases, and discusses the properties and regulation of the kinases and phosphatases responsible for catalyzing the phosphorylation-dephosphorylation of these substrates. In addition to the regulatory and enzymatic proteins associated with deoxyribonucleic acid transcription and replication, several other nuclear enzymes and proteins have been shown to be subject to phosphorylation by protein kinase. Phosphorylated nonhistone proteins have been detected in chromatin fractions prepared by a wide variety of procedures. The High Mobility Group proteins, a group of relatively small nonhistone proteins characterized by high electrophoretic mobility in low pH polyacrylamide gels. The heterogeneous nuclear Ribonucleic acid (RNA) molecules, which are the precursors of messenger RNA in eukaryotic cells, are associated with a mixture of nonhistone proteins to form ribonucleoprotein particles. The most extensively studied substrate for nuclear protein kinase is clearly histone H1, the largest and most variable of the histones.
