The Catalytic Subunit of cAMP-Dependent Protein Kinase from Ascaris suum

The Cloning and Structure of a Novel Subtype of Protein Kinase A

A complete cDNA clone encoding the catalytic subunit of cAMP-dependent protein kinase of Ascaris suum was constructed from two overlapping partial clones. The encoded sequence of 337 ammo acids is 48 % identical with the sequence of mouse Cα subunit. Approximately the same low similarity was found with the sequence of the C subunit from another nematode, Caenorhabditis elegans. The N-terminal 14 amino acids and the myristoylation site of the mammalian protein are not contained in the enzyme from Ascaris. Two cysteines (Cys33 and Cys319) replace a basic residue in the N-terminal region and an acidic amino acid near the C-terminus which are conserved in all known C subunits from other sources. The substitutions provide the possibility of disulfide bridge formation between the N-terminal and C-terminal parts of the protein. There is strong evidence that a single gene encodes cAMP-dependent protein kinase in Ascaris. Modelling of the sequence into the coordinates of the X-ray structure of the mammalian enzyme suggest a high degree of conservation in the three-dimensional structure. However, structural variations occur at the surface of the protein near the catalytic cleft and are likely to account for the variations in substrate specificity previously observed between the purified protein kinase from Ascaris [Thalhofer, H. P., Daum, G., Harris, B. G. & Hofer, H. W. (1988) J. Biol. Chem. 263, 952–957] and the mammalian enzyme.