AN UNUSUAL CATALYTIC SUBUNIT FOR THE CAMP-DEPENDENT PROTEIN-KINASE OFDICTYOSTELIUM-DISCOIDEUM

The cAMP-dependent protein kinase (cAPK) plays an essential role durin g differentiation and fruit morphogenenesis in Dictyostelium discoideu m. The presence of an open reading frame on the gene, pkaC (previously named either Dd PK2 or Dd PK3 by different groups), predicts a 73-kDa polypeptide with 54% similarity to the catalytic subunits of cAPKs fr om other organisms. Using anti-peptide antibodies, we show that the pk aC gene product, PkaC, is a 73-kDa polypeptide. Despite the fact that PkaC is about twice the size of its mammalian counterparts, it possess es all of the properties required of a catalytic subunit. It is physic ally associated with the regulatory subunit, and this association resu lts in an inhibition of the catalytic activity which is reverted by cA MP. PkaC copurifies with cAPK activity, and an increased cAPK activity is observed in cells overexpressing PkaC. We conclude that PkaC is a catalytic subunit of the Dictyostelium discoideum cAPK and discuss the unusual features of this protein with the highest molecular weight of known cAPKs.