Compensatory Regulation of RIα Protein Levels in Protein Kinase A Mutant Mice

Abstract The cAMP-dependent protein kinase holoenzyme is assembled from regulatory (R) and catalytic (C) subunits that are expressed in tissue-specific patterns. Despite the dispersion of the R and C subunit genes to different chromosomal loci, mechanisms exist that coordinately regulate the intracellular levels of R and C protein such that cAMP-dependent regulation is preserved. We have created null mutations in the RIβ and RIIβ regulatory subunit genes in mice, and find that both result in an increase in the level of RIα protein in tissues that normally express the β isoforms. Examination of RIα mRNA levels and the rates of RIα protein synthesis in wild type and RIIβ mutant mice reveals that the mechanism of this biochemical compensation by RIα does not involve transcriptional or translational control. These in vivo findings are consistent with observations made in cell culture, where we demonstrate that the overexpression of Cα in NIH 3T3 cells results in increased RIα protein without increases in the rate of RIα synthesis or the level of RIα mRNA. Pulse-chase experiments reveal a 4-5-fold increase in the half-life of RIα protein as it becomes incorporated into the holoenzyme. Compensation by RIα stabilization may represent an important biological mechanism that safeguards cells from unregulated catalytic subunit activity.