Aurothiomalate Inhibits Transformed Growth by Targeting the PB1 Domain of Protein Kinase Cι

Abstract We recently identified the gold compound aurothiomalate (ATM) as a potent inhibitor of the Phox and Bem1p (PB1)-PB1 domain interaction between protein kinase C (PKC) ι and the adaptor molecule Par6. ATM also blocks oncogenic PKCι signaling and the transformed growth of human lung cancer cells. Here we demonstrate that ATM is a highly selective inhibitor of PB1-PB1 domain interactions between PKCι and the two adaptors Par6 and p62. ATM has no appreciable inhibitory effect on other PB1-PB1 domain interactions, including p62-p62, p62-NBR1, and MEKK3-MEK5 interactions. ATM can form thio-gold adducts with cysteine residues on target proteins. Interestingly, PKCι (and PKCζ) contains a unique cysteine residue, Cys-69, within its PB1 domain that is not present in other PB1 domain containing proteins. Cys-69 resides within the OPR, PC, and AID motif of PKCι at the binding interface between PKCι and Par6 where it interacts with Arg-28 on Par6. Molecular modeling predicts formation of a cysteinyl-aurothiomalate adduct at Cys-69 that protrudes into the binding cleft normally occupied by Par6, providing a plausible structural explanation for ATM inhibition. Mutation of Cys-69 of PKCι to isoleucine or valine, residues frequently found at this position in other PB1 domains, has little or no effect on the affinity of PKCι for Par6 but confers resistance to ATM-mediated inhibition of Par6 binding. Expression of the PKCι C69I mutant in human non-small cell lung cancer cells confers resistance to the inhibitory effects of ATM on transformed growth. We conclude that ATM inhibits cellular transformation by selectively targeting Cys-69 within the PB1 domain of PKCι.