Crystal structure of the 14-3-35 zeta : serotonin N-acetyltransferase complex: A role for scaffolding in enzyme regulation

Serotonin N-acetyltransferase (AANAT) controls the daily rhythm in melatoni n synthesis. When isolated from tissue, AANAT copurifies with isoforms epsi lon and zeta of 14-3-3. We have determined the structure of AANAT bound to 14-3-3 zeta, an association that is phosphorylation dependent. AANAT is bou nd in the central channel of the 14-3-3 zeta dimer, and is held in place by extensive interactions both with the amphipathic phosphopeptide binding gr oove of 14-3-3 zeta and with other parts of the central channel. Thermodyna mic and activity measurements, together with crystallographic analysis, ind icate that binding of AANAT by 14-3-3 zeta modulates AANAT's activity and a ffinity for its substrates by stabilizing a region of AANAT involved in sub strate binding.