Thebform of human cAMP-dependent protein kinase inhibitor (human PKIb), a novel heat-stable protein, was isolated with high yield using a bacterial expression system. Assays of PKI activity demonstrated that purified PKIb inhibits the catalytic subunit of cAMP-dependent protein kinase. FTIR, Raman spectroscopy and CD experiments implied that human PKIbcontained only small amounts of a-helix andb-structures, but large amounts of random coil and turn structures, which may explain its high thermosta-bility. The details of its conformational changes in response to heat were studied by CD experiments for the first time, revealing that the protein unfolded at high temperature and refolded when decreased to room temperature
