A-Kinase-Interacting Protein Localizes Protein Kinase A in the Nucleus

The genetic variability and covalent modifications associated with the amino terminus of the protein kinase A (PKA) catalytic (C) subunit suggest that it may contribute to protein-protein interactions and/or localization. By using a yeast two-hybrid screen, we identified a PKA-interacting protein (AKIP1) that binds to the amino terminus (residues 1-39) of the C subunit of PKA. The interaction was localized to the A helix (residues 14-39) of the C subunit and to the carboxyl terminus of AKIP1. AKIP1 thus defines the amino-terminal A helix of PKA as a protein interaction motif. In normal breast (Hs 578 Bst) and HeLa cells, AKIP1 is present in the nucleus as speckles. A nuclear localization signal (Arg-14 and Arg-15) was identified. Upon stimulation with forskolin, HeLa cells expressing AKIP1 accumulated higher levels of the endogenous C subunit in the nucleus. Deletion of the carboxyl terminus of AKIP1 or overexpression of residues 1-39 of the C subunit abolished nuclear localization of the activated endogenous C subunit. Thus, AKIP1 describes a PKA-interacting protein that can contribute to localization by a mechanism that is distinct from A-kinase anchoring proteins that interact with the regulatory subunits.