CAMP-DEPENDENT PROTEIN-KINASE DEFINES A FAMILY OF ENZYMES

The structure of the recombinant mouse catalytic subunit of cAMP-depen dent protein kinase is reviewed with particular emphasis on the overal l features and specific amino acids that are shared by all members of the eukaryotic protein kinase family. The crystal structure of a terna ry complex containing both MgATP and a twenty-residue inhibitor peptid e defines the precise role of the conserved residues that are clustere d at the active site. In addition to catalysing the post-translational modification of other proteins, the catalytic subunit is itself subje ct to covalent modifications. It is a phosphoprotein and is also myris tylated at its amino terminus. The enzyme when crystallized in the pre sence of detergent shows a detergent molecule bound to an acyl pocket that is presumably occupied by the myristyl moiety in the mammalian en zyme. When expressed in E. coli, the catalytic subunit is autophosphor ylated at four sites. Two stable phosphates at Ser338 and Thr197 inter act with multiple protein side chains thus explaining why they are ina ccessible to phosphatases. Although all substrates and inhibitors of t he catalytic subunit share a general minimum consensus sequence, the h igh affinity binding of protein inhibitors such as the regulatory subu nits and the heat stable protein kinase inhibitors require additional determinants that the beyond the consensus site. These two physiologic al inhibitors of the catalytic subunit appear to use different sites t o achieve high-affinity binding.