Binding of [26-³H]bryostatin 1 and analogs to calcium-dependent and calcium-independent protein kinase C isozymes

In this study we explored the pattern of protein kinase C (PKC) isozyme selectivity of the bryostatins, a unique class of PKC activators that induce only a subset of the typical phorbol ester responses and antagonize those phorbol ester-mediated responses that they themselves fail to induce. The binding properties of individual recombinant PKC isozymes that had been expressed in insect cells, isolated, and reconstituted in Triton X- 100/phosphatidylserine mixed micelles were determined. [³H]Bryostatin 1 showed lower affinity for PKC-β₁ and -γ, compared with PKC-α, -δ, -ε, and η. This pattern contrasts with that observed for other PKC ligands. These latter assays were conducted with isozymes reconstituted in phosphatidylserine, conditions that unfortunately do not permit quantitation of bryostatin 1 binding under equilibrium conditions. Using Δ^19,20- bryostatin 10 and Δ^19,20-isobryostatin 10, we could distinguish the respective roles of ligand and lipid in the pattern of selectivity. When isozymes were reconstituted in phosphatidylserine vesicles, Δ^19,20- bryostatin 10 and Δ^19,20-isobryostatin 10 showed similar affinities for PKC-α and -γ, similarly to the phorbol esters. However, in the mixed micellar system, PKC-γ showed a significantly lower binding affinity, as had been observed for bryostatin 1. These results suggest that the unique pattern of biological responses to the bryostatins does not represent a unique pattern of isotype recognition. Furthermore, the lipid environment of PKC plays an important role in determining the binding selectivity for individual isozymes.