Expression of the type I regulatory subunit of cAMP-dependent protein kinase in Escherichia coli

The cDNA for the bovine type I regulatory subunit of cAMP-dependent protein kinase has been inserted into the expression vector pUC7. When E. coli JM105 was transformed with this plasmid, R-subunit was expressed in amounts that approached 2-4 mg/liter. The expressed protein was visualized in total cell extracts by photolabeling with 8-N3-[32P]-cAMP following transfer from SDS polyacrylamide gels to nitrocellulose. Expression of R-subunit was independent of IPTG. R-subunit accumulated in large amounts only in the stationary phase of growth. The addition of IPTG during the log phase of growth actually blocked the accumulation of R-subunit. The soluble, dimeric R-subunit was purifided to homogeneity by affinity chromatography. This R-subunit bound 2 mol cAMP/mol R monomer, reassociated with C-subunit to form cAMP-dependent holoenzyme, and migrated as a dimer on SDS polyacrylamide gels in the absence of reducing agents. The expressed protein was also susceptible to limited proteolysis yielding a monomeric cAMP-binding fragment having a molecular weight of 35,000. In all of these properties the expressed protein was indistinguishable from R/sup I/ purified from bovine tissue even though the R-subunit expressed in E. coli represents a fusion protein that contains 10 additional amino acids at the amino terminus that are provided by the lac Z gene of the vector. The NH2-terminal sequence was confirmed by amino acid sequencing