Cellular localization and kinetics of the rice melatonin biosynthetic enzymes SNAT and ASMT

Authors

  • Yeong Byeon,

    1. Department of Biotechnology, Interdisciplinary Program of Bioenergy and Biomaterials, Bioenergy Research Center, Chonnam National University, Gwangju, Korea
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  • Hyoung Yool Lee,

    1. Department of Biotechnology, Interdisciplinary Program of Bioenergy and Biomaterials, Bioenergy Research Center, Chonnam National University, Gwangju, Korea
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  • Kyungjin Lee,

    1. Department of Biotechnology, Interdisciplinary Program of Bioenergy and Biomaterials, Bioenergy Research Center, Chonnam National University, Gwangju, Korea
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  • Sangkyu Park,

    1. Department of Biotechnology, Interdisciplinary Program of Bioenergy and Biomaterials, Bioenergy Research Center, Chonnam National University, Gwangju, Korea
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  • Kyoungwhan Back

    Corresponding author
    1. Department of Biotechnology, Interdisciplinary Program of Bioenergy and Biomaterials, Bioenergy Research Center, Chonnam National University, Gwangju, Korea
    • Address reprint requests to Kyoungwhan Back, Department of Biotechnology, College of Agriculture and Life Sciences, Chonnam National University, Gwangju 500-757, South Korea.

      E-mail: kback@chonnam.ac.kr

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Abstract

Serotonin N-acetyltransferase (SNAT) and N-acetylserotonin methyltransferase (ASMT) are the final two enzymes in the melatonin synthesis pathway in plants. Although their corresponding genes have been cloned, their cellular localization and enzymatic characteristics are unknown. Using confocal microscopy, we showed that SNAT protein is localized in chloroplasts, whereas ASMT is expressed in the cytoplasm. In vitro measurement of ASMT enzyme activity revealed a peak of activity in roots, but SNAT enzyme activity was not detected in any plant tissues. This may be attributed in part to an effect of chlorophyll because SNAT enzyme activity was greatly inhibited by chlorophyll in a dose-dependent manner. Because the SNAT protein of cyanobacteria is thermophilic, we examined the effect of temperature on the activity of the rice SNAT and ASMT enzymes. Purified recombinant rice SNAT and ASMT enzymes had an optimum temperature for activity of 55°C. The Km and Vmax values for SNAT at 55°C were 270 μm and 3.3 nmol/min/mg protein, whereas the Km and Vmax for ASMT were 222 μm and 9 nmol/min/mg protein, respectively. The catalytic efficiency (Vmax/Km) values of SNAT and ASMT were 16-fold and 4054-fold higher at 55°C than at 30°C suggestive of increased melatonin production at high temperature in plants.

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