ABSTRACT
cAMP-dependent protein kinases (PKA) are the primary mediators of cAMP action, binding of cAMP leading to the dissociation of an inactive tetrameric enzyme into a dimer of regulatory (R) subunits and two active catalytic (C) subunit monomers. The catalytic subunits then phosphorylate specific protein substrates, on serine and threonine residues, thereby altering the biochemical properties of these proteins. Changes in cAMP-dependent protein kinase levels have been reported in mammalian cells during differentiation and development, during progression through the cell cycle, and in transformed cells, suggesting a role for PKA in these processes. In lower eukaryotes similar results have been reported. The veast S. cerevisiae for instance, requires correct regulation of cAMP-dependent protein kinase activity for normal progression through the cell cycle, sporulation and starvation-induced growth arrest. Furrthermore, regulatory subunit levels increase 8-fold in stationary-phase yeast cells. In the slime mould D. Discoideum and the aquatic fungus B. Emersonii, nutrient starvation induces cell differentiation and development, and a drastic increase in cAMP-dependent protein kinase subunit levels is observed during these processes.