Controls the night/day rhythm of melatonin production in the pineal gland. Catalyzes the N-acetylation of serotonin into N-acetylserotonin, the penultimate step in the synthesis of melatonin. 2-arylethylamine + acetyl-CoA = CoA + H(+) + N-acetyl-2-arylethylamine Aromatic compound metabolism; melatonin biosynthesis; melatonin from serotonin: step 1/2. Monomer. Interacts with several 14-3-3 proteins, including YWHAB, YWHAE, YWHAG and YWHAZ, preferentially when phosphorylated at Thr-31. Phosphorylation on Ser-205 also allows binding to YWHAZ, but with a 10-fold lower affinity. The interaction with YWHAZ considerably increases affinity for arylalkylamines and acetyl-CoA and protects the enzyme from dephosphorylation and proteasomal degradation. It may also prevent thiol-dependent inactivation. The physiological stoichiometry of the interaction is not clear. In vitro studies show either 1:2 (i.e. 1 AANAT molecule per YWHAZ dimer) (PubMed:11427721) or 2:2 (PubMed:11336675). Highest expression in the pineal gland, followed by retina. Expressed at much lower levels in brainstem and pituitary gland. AANAT activity also detected at low levels in the olfactory lobe. Exhibits night/day variations with a 7-fold increased activity at night. At the mRNA level, the nocturnal increase is lower than 2-fold. cAMP-dependent phosphorylation on both N-terminal Thr-31 and C-terminal Ser-205 regulates AANAT activity by promoting interaction with 14-3-3 proteins. Belongs to the acetyltransferase family. AANAT subfamily.