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Published on August 31, 2007; 10.1104/pp.107.104679

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Received June 26, 2007
Accepted August 23, 2007

The Import of S-adenosyl Methionine into the Golgi Apparatus Is Required for the Methylation of Homogalacturonan

Consuelo Ibar and Ariel Orellana *

Millenium Nucleus in Plant Cell Biology, Center of Plant Biotechnology, Andrés Bello University, República 217, Santiago, Chile

* Corresponding author; email: aorellana{at}unab.cl.

S-adenosylmethionine (SAM) is the substrate used in the methylation of homogalacturonan (HGA) in the Golgi apparatus. SAM is synthesized in the cytosol but it is not currently known how it is then transported into the Golgi. In our current study , we find that homogalacturonan methyltransferase (HGA-MT) is present in Golgi-enriched fractions and that its catalytic domain faces the lumen of this organelle. This suggests that SAM must be imported into the Golgi. We performed uptake experiments using [methyl-14C]-SAM and found that SAM is incorporated into the Golgi vesicles resulting in the methylation of polymers that are sensitive to pectinase and pectin methylesterase, but not to proteases. To avoid detecting the transfer reaction, we also used [carboxyl-14C]-SAM, the uptake of which into Golgi vesicles was found to be sensitive to temperature, detergents, osmotic changes and to be saturable with a Km of 33 µM. Double label uptake experiments using [methyl-3H]-SAM and [carboxyl-14C]-SAM also revealed a time-dependent increase in the 3H/14C ratio, suggesting that upon transfer of the methyl group, the resulting S-adenosylhomocysteine (SAH) is not accumulated in the Golgi. SAM incorporation was also found to be inhibited by S-adenosylhomocysteine, whereas UDP-GalA, UDP-GlcA and AcetylCoA had no effect. DIDS, a compound that inhibits nucleotide sugar transporters, also had little effect upon SAM incorporation. Interestingly, the combination of UDP-GalA + Acetyl-CoA or UDP-GlcA + Acetyl-CoA produced a slight increase in the uptake of SAM. These results support the idea that a SAM transporter is required for HGA biosynthesis.






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