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. Cyclic GMP-activated protein kinases capable of catalyzing the phosphorylation of histones as well as nonhistones have been detected in nuclei, as have cyclic GMP-binding proteins, guanylate cyclase, and cyclic GMP itself. The chapter discusses the properties of two kinds of nonhistone phosphoproteins which may regulate transcription isolated from the nucleolus and from the nervous tissue. A multitude of nuclear protein kinases appear to be involved in catalyzing the phosphorylation of these various substrates, though the exact number is uncertain at present because of the difficulty in purifying these enzymes to homogeneity. Many factors have been implicated in the regulation of nuclear protein kinase activity, including ions, polyamines, cyclic nucleotides, nuclear translocation of cytoplasmic protein kinase, protein kinase autophosphorylation, and protein kinase inhibitors.
