Molecular characterization of protein kinase C delta (PKCδ)-Smac interactions

Background: Protein kinase C delta (PKC delta) is known to be an important regulator of apoptosis, having mainly pro-but also anti-apoptotic effects depending on context. In a previous study, we found that PKC delta interacts with the pro-apoptotic protein Smac. Smac facilitates apoptosis by suppressing inhibitor of apoptosis proteins (IAPs). We previously established that the PKC delta-Smac complex dissociates during induction of apoptosis indicating a functional importance. Because the knowledge on the molecular determinants of the interaction is limited, we aimed at characterizing the interactions between PKC delta and Smac. Results: We found that PKC delta binds directly to Smac through its regulatory domain. The interaction is enhanced by the PKC activator TPA and seems to be independent of PKC delta catalytic activity since the PKC kinase inhibitor GF109203X did not inhibit the interaction. In addition, we found that C1 and C2 domains from several PKC isoforms have Smac-binding capacity. Conclusions: Our data demonstrate that the Smac-PKC delta interaction is direct and that it is facilitated by an open conformation of PKC delta. The binding is mediated via the PKC delta regulatory domain and both the C1 and C2 domains have Smac-binding capacity. With this study we thereby provide molecular information on an interaction between two apoptosis-regulating proteins.