Human Naa50 Shows Serotonin -Acetyltransferase Activity, and Its Overexpression Enhances Melatonin Biosynthesis, Resulting in Osmotic Stress Tolerance in Rice.

A new clade of serotonin -acetyltransferase (SNAT), the penultimate enzyme in the melatonin biosynthetic pathway, has been reported in the archaeon . The closest homolog of archaea SNAT in human was an -alpha-acetyltransferase50 (Naa50). To determine whether human Naa50 (hNaa50) shows SNAT enzyme activity, we chemically synthesized and expressed the gene in , followed by Ni affinity purification. Purified recombinant hNaa50 showed SNAT activity ( and values of 986 μM and 1800 pmol/min/mg protein, respectively). To assess its in vivo function, was overexpressed in rice (OE). The transgenic rice plants produced more melatonin than nontransgenic wild-type rice, indicating that is functionally coupled with melatonin biosynthesis. Due to its overproduction of melatonin, -OE had a higher tolerance against osmotic stress than the wild type. Enhanced expression of the chaperone genes and in -OE plants was responsible for the increased tolerance. It is concluded that harbors serotonin -acetyltransferase enzyme activity in addition to its initial -alpha-acetyltransferase, suggesting the bifunctionality of the enzyme toward serotonin and protein substrates. Consequently, ectopic overexpression of in rice enhanced melatonin synthesis, indicating that is in fact involved in melatonin biosynthesis.