Compensatory regulation of RI alpha protein levels in protein kinase A mutant mice

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 RI beta and RII beta regulatory subunit genes in mice, and find that both result in an increase in the level of RI alpha protein in tissues that normally express the beta isoforms. Examination of RI alpha mRNA levels and the rates of RI alpha protein synthesis in wild type and RII beta mutant mice reveals that the mechanism of this biochemical compensation by RI alpha 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 C alpha in NIH 3T3 cells results in increased RI alpha protein without increases in the rate of RI alpha synthesis or the level of RI alpha mRNA. Pulse-chase experiments reveal a 4-5-fold increase in the half-life of RI alpha protein as it becomes incorporated into the holoenzyme, Compensation by RI alpha stabilization may represent an important biological mechanism that safeguards cells hom unregulated catalytic subunit activity.