Localization of Triton-insoluble cAMP-dependent kinase type RIbeta in rat and mouse brain

In eukaryotic cells, cAMP regulates many different cellular functions. Its effects are in most cases mediated by cAMP-dependent protein kinases. These consist of two regulatory and two catalytic subunits. In mammals, four different isoforms of cAMP-dependent protein kinases regulatory subunits have been characterized (RIalpha and beta, RIIalpha and beta). These four isoforms show a high level of homology and slightly different biochemical properties. In addition to biochemical properties, a different anatomical distribution of the regulatory isoforms may contribute to determine the specificity of diverse cAMP effects. By immunohistochemistry, the distribution of the detergent-insoluble fraction of RIbeta isoform has been examined in rat and mouse brain. Biochemical fractionation shows that a large fraction of both RIalpha and RIbeta isoforms is bound to the cytoskeleton. RIbeta labelling can be observed only in few locations: Purkinje cells, olfactory mitral cells, lateral thalamic neurons, superior olivary complex neurons. These cell populations are involved in the so called Purkinje cell degeneration. On the other hand, RIalpha aggregates have a more widespread distribution, in brain areas involved in visceroemotional control. At the subcellular level, these two subunits show a different pattern of labelling: in most cells a sharply defined clustered labelling is observed for RIalpha isoforms, while the RIbeta isoform presents a weaker, diffuse intracytoplasmic distribution. Competition experiments point to the presence of, as yet unidentified, different and selective anchoring proteins for the two similar RIalpha and beta isoforms. It is suggested that, as is the case for structural proteins, a different supramolecular organization of similar regulatory proteins may be crucial in order to fulfill different functions.