THE CATALYTIC SUBUNIT OF DICTYOSTELIUM CAMP-DEPENDENT PROTEIN-KINASE - ROLE OF THE N-TERMINAL DOMAIN AND OF THE C-TERMINAL RESIDUES IN CATALYTIC ACTIVITY AND STABILITY

The C subunit of Dictyostelium cAMP-dependent protein kinase (PKA) is unusually large (73 kDa) due to the presence of 330 amino acids N-term inal to the conserved catalytic core. the sequence following the core, including a C-terminal -Phe-Xaa-Xaa-Phe-COOH motif, is highly conserv ed. We have characterized the catalytic activity and stability of C su bunits mutated in sequences outside the catalytic core and we have ana lyzed their ability to interact with the R subunit and with the heat-s table protein-kinase inhibitor PKI. Mutants carrying deletions in the N-terminal domain displayed little difference in their kinetic propert ies and retained their capacity to be inhibited by R subunit and by PK I. In contrast, the mutation of one or both of the phenylalanine resid ues in the C-terminal motif resulted in a decrease of catalytic activi ty and stability of the proteins. Inhibition by the R subunit or by PK I were however unaffected. Sequence-comparison analysis of other prote in kinases revealed that a -Phe-Xaa-Xaa-Phe- motif is present in many Ser/Thr protein kinases, although its location at the very end of the polypeptide is a particular feature of the PKA family. We propose that the presence of this motif may serve to identify isoforms of protein kinases.