Two Well-defined Motifs in the CAMP-dependent Protein Kinase Inhibitor (PKIalpha) Correlate with Inhibitory and Nuclear Export Function

The heat stable inhibitor of cAMP-dependent protein kinase (PKIalpha) contains both a nuclear export signal (NES) and a high affinity inhibitory region that is essential for inhibition of the catalytic subunit of the kinase. These functions are sequentially independent. Two-dimensional NMR spectroscopy was performed on uniformly [15N]-labeled PKIalpha to examine its structure free in solution. Seventy out of 75 residues were identified, and examination of the CaH chemical shifts revealed two regions of upfield chemical shifts characteristic of alpha-helices. When PKIalpha was fragmented into two functionally distinct peptides for study at higher concentrations, no significant alterations in chemical shifts or secondary structure were observed. The first ordered region, identified in PKIalpha (1-25), contains an alpha-helix from residues 1-13. This helix extends by one turn the helix observed in the crystal structure of a PKIalpha (5-24) peptide bound to the catalytic subunit. The second region of well-defined secondary structure, residues 35-47, overlaps with the nuclear export signal in the PKIalpha (26-75) fragment. This secondary structure consists of a helix with a hydrophobic face comprised of Leu37, Leu41, and Leu44, followed by a flexible turn containing Ile46. These four residues are critical for nuclear export function. The remainder of the protein in solution appears relatively unstructured, and this lack of structure surrounding a few essential and well-defined signaling elements may be characteristic of a growing family of small regulatory proteins that interact with protein kinases.