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Published on July 20, 2007; 10.1104/pp.107.101162

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Received April 17, 2007
Accepted July 10, 2007

Enzymatic Properties and Subcellular Localization of Arabidopsis Beta-N-acetylhexosaminidases

Richard Strasser *, Jayakumar Singh Bondili , Jennifer Schoberer , Barbara Svoboda , Eva Liebminger , Josef Glössl , Friedrich Altmann , Herta Steinkellner , and Lukas Mach

Institute of Applied Genetics and Cell Biology, BOKU, University of Natural Resources and Applied Life Sciences, Muthgasse 18, A-1190 Vienna, Austria; Department of Chemistry, BOKU, University of Natural Resources and Applied Life Sciences, Muthgasse 18, A-1190 Vienna, Austria

* Corresponding author; email: richard.strasser{at}boku.ac.at.

Plant glycoproteins contain substantial amounts of paucimannosidic N-glycans lacking terminal GlcNAc residues at their non-reducing ends. It has been proposed that this is due to the action of {beta}-hexosaminidases during late stages of N-glycan processing or in the course of N-glycan turnover. We have now cloned the three putative {beta}-hexosaminidase sequences present in the Arabidopsis thaliana genome. When heterologously expressed as soluble forms in Spodoptera frugiperda cells, the enzymes (termed HEXO1-3) could all hydrolyze the synthetic substrates pNP-GlcNAc, pNP-GalNAc, MU-GlcNAc and MU-GlcNAc-6SO4, albeit to a varying extent. HEXO1-3 were further able to degrade chitotriose-PA, whereas chitobiose-PA was only cleaved by HEXO1. With N-glycan substrates, HEXO1 displayed a much higher specific activity than HEXO2 and HEXO3. Nevertheless, all three enzymes were capable of removing terminal GlcNAc residues from the {alpha}1,3- and {alpha}1,6-mannosyl branches of biantennary N-glycans without any strict branch preference. Subcellular localization studies with HEXO-fluorescent protein fusions transiently expressed in Nicotiana benthamiana plants showed that HEXO1 is a vacuolar protein. In contrast, HEXO2 and HEXO3 are mainly located at the plasma membrane. These results indicate that HEXO1 participates in N-glycan trimming in the vacuole, whereas HEXO2 and/or HEXO3 could be responsible for the processing of N-glycans present on secretory glycoproteins.




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