Ultrastructural localization of protein kinase subunits in glucagon-treated rat hepatocytes applying the colloidal-gold technique

Abstract

The molecular mechanisms by which non-steroidal hormones regulate transcription in eukaryotes has not been elucidated. However, cyclic AMP, an important second messenger of the hormonal signal has been suggested to play a significant role in the regulation of eukaryotic gene expression (1). It has also been proposed that hormones functioning through the cAMP/adenylate cyclase system initiate a series of biochemical events in the target tissue resulting in the synthesis of new RNA and protein (2). The important link in this scheme appears to be the cAMP dependent protein kinase which may translocate from cytoplasm to nucleus following its activation by cAMP. Two types of soluble cAMP dependent protein kinases exist in mammalian tissues (type I and II). Each isozyme exists as a tetramer composed of two catalytic (C) and two regulatory (R) subunits. Binding of cAMP to the R subunit activates the C subunit according to the following stoichiometry: R2C2 + 4cAMP = R2·cAMP4 + 2C.