Phosphatidic Acid‐Dependent Phosphorylation of a 29‐kDa Protein by Protein Kinase Cα in Bovine Brain Cytosol

: Activation of phospholipase D (PLD) is involved in receptor‐mediated signal transduction responses. Signaling from PLD to a downstream molecule(s) appears to be mediated by the PLD product phosphatidic acid (PA). A target molecule(s) of PA, however, has not yet been identified. The present study sought to define such a target molecule(s) of PA. In bovine brain cytosol, proteins with apparent molecular weights of 29,000 (p29) and 32,000 (p32) were prominently phosphorylated in the presence of PA, but not in its absence, indicating that there is a PA‐regulated protein kinase (PARK) in bovine brain that phosphorylates p29 and p32. One of these substrates, p29, was purified to near homogeneity. Its partial amino acid sequence was determined and found to be identical to that of a known brain‐specific 25‐kDa protein (p25). The purified p29 was also readily recognized by and immunoprecipitated with an anti‐p25 antibody. These results suggest that p29 is very similar to or identical with p25. Using the purified p29 as a substrate, PARK was purified to near homogeneity. The purified PARK had an apparent molecular weight of 80,000, was strongly recognized by an anti‐protein kinase C (PKC)α antibody, and was activated by phosphatidylserine (PS) as well as PA. The PA‐ and PS‐stimulated PARK activity was extremely augmented by the presence of 1 µM free Ca2+. In the presence of 1 mM EGTA, phorbol 12‐myristate 13‐acetate activated PARK synergistically with PA or PS. Similar results were obtained with the purified recombinant PKCα. From these results, it is suggested that the PARK activity purified might be attributed to PKCα. In p25‐depleted bovine brain cytosol, which was prepared by treatment of bovine brain cytosol with the anti‐p25 antibody, PA‐dependent phosphorylation of p29, but not p32, was almost completely eliminated. When PKCα in bovine brain cytosol was depleted by its precipitation with the anti‐PKCα antibody, neither p29 nor p32 in this PKCα‐depleted cytosol was phosphorylated in the presence of PA. These results indicate that in bovine brain cytosol PA activates PKCα, which, in turn, phosphorylates p29, which may be identical with p25.