14-3-3 proteins associate with A20 in an isoform-specific manner and function both as chaperone and adapter molecules

A20, a novel zinc finger protein, is an inhibitor of tumor necrosis factor-induced apoptosis. The mechanism by which A20 exerts its protective effect is currently unknown. Several isoforms of the 14-3-3 proteins were found to interact with A20 in a yeast two-hybrid screen, A20 bound several 14-3-3 isoforms in vitro. Moreover, transfected A20 was found to preferentially bind the endogenous eta 14-3-3 isoform, whereas the beta/zeta isoforms co-immunoprecipitated much less efficiently, and epsilon 14-3-3 had an intermediate affinity. Importantly, c-Raf, a previously described 14-3-3-interacting protein, also preferentially bound the eta isoform. The cellular localization and subcellular fractionation of A20 was dramatically altered by co-transfected 14-3-3, providing the first experimental evidence for the notion that 14-3-3 can function as a chaperone, Furthermore, c-Raf and A20 co-immunoprecipitated in a 14-3-3-dependent manner, suggesting that 14-3-3 can function as a bridging or adapter molecule.