Catalyzes the N-acetylation of serotonin into N-acetylserotonin. 2-arylethylamine + acetyl-CoA = CoA + H(+) + N-acetyl-2-arylethylamine Aromatic compound metabolism; melatonin biosynthesis; melatonin from serotonin: step 1/2. Monomer (By similarity). Interacts with YWHAZ; the interaction requires phosphorylation on Thr-29, and modulates the enzymatic activity of AANAT through preventing dephosphorylation and/or proteolysis and stabilizing substrate binding. Subsequently, a second molecule of AANAT can bind, via the phosphorylated Ser-203 site, the other ywhaz monomer with similar effect (By similarity). Expressed in follicular pineal cells and retinal photoreceptors with five times more expression in the pineal gland. Exhibits circadian rhythm pattern in the pineal gland with highest levels at ZT 24. Expression in the retinal photoreceptor cells oscillates in a circadian manner. Phosphorylated; cAMP-dependent phosphorylation on both N-terminal and C-terminal sites regulates AANAT activity through allowing interaction with 14-3-3 proteins and protecting the enzyme against proteasomal degradation. Belongs to the acetyltransferase family. AANAT subfamily.