Fig. 2. PKA phosphorylates apSyn at a single site in domain A. (A) Radioactive phosphate incorporation into recombinant apSyn 11.1 or bovine synapsin I (5 pmol) as a function of the incubation time demonstrates that the snail synapsin orthologue is an excellent substrate for PKA. The multiple bands observed in the apSyn lanes are probably caused by partial cleavage by thrombin. (B) Mutation of Ser-9 in the conserved PKA consensus sequence present in apSyn domain A into alanine (apSynALA9) or aspartate (apSynASP9) completely abolishes [³²P] incorporation after incubation with the PKA catalytic subunit for 20 minutes, indicating that Ser-9 is the only phosphorylation site for PKA. The positions of molecular mass standards are shown on the left in kDa.