CHARACTERIZATION OF PKI-GAMMA, A NOVEL ISOFORM OF THE PROTEIN-KINASE INHIBITOR OF CAMP-DEPENDENT PROTEIN-KINASE

Attempts to understand the physiological roles of the protein kinase i nhibitor (PKI) proteins have been hampered by a lack of knowledge conc erning the molecular heterogeneity of the PKI family. The PKI gamma cD NA sequence determined here predicted an open reading frame of 75 amin o acids, showing 35% identity to PKI alpha and 30% identity to PKI bet a 1. Residues important for the high affinity of PKI alpha and PKI bet a 1 as well as nuclear export of the catalytic (C) subunit of cAMP-dep endent protein kinase were found to be conserved in PKI gamma. Norther n blot analysis showed that a 1.3-kilobase PKI gamma message is widely expressed, with highest levels in heart, skeletal muscle, and testis. RNase protection analysis revealed that in most tissues examined PKI gamma is expressed at levels equal to or higher than the other known P KI isoforms and that in several mouse-derived cell lines, PKI gamma is the predominant PKI message. Partial purification of PKI activities f rom mouse heart by DEAE ion exchange chromatography resolved two major inhibitory peaks, and isoform-specific polyclonal antibodies raised a gainst recombinant PKI alpha and PKI gamma identified these inhibitory activities to be PKI alpha and PKI gamma. A comparison of inhibitory potencies of PKI alpha and PKI gamma expressed in Escherichia coil rev ealed that PKI gamma was a potent competitive inhibitor of C alpha pho sphotransferase activity in vitro (K-i = 0.44 nM) but is 6-fold less p otent than PKI alpha (K-i = 0.073 nM). Like PKI alpha, PKI gamma was c apable of blocking the nuclear accumulation of Flag-tagged C subunit i n transiently transfected mammalian cells. Finally, the murine PKI gam ma gene was found to overlap the murine adenosine deaminase gene on mo use chromosome 2. These results demonstrate that PKI gamma is a novel, functional PKI isoform that accounts for the previously observed disc repancy between PKI activity and PKI mRNA levels in several mammalian tissues.