title: Arabinogalactan glycosyltransferases target to a unique subcellular compartment that may function in unconventional secretion in plants.
abstract: We report that fluorescently tagged arabinogalactan glycosyltransferases target not only the Golgi apparatus but also uncharacterized smaller compartments when transiently expressed in Nicotiana benthamiana. Approximately 80% of AtGALT31A [Arabidopsis thaliana galactosyltransferase from family 31 (At1g32930)] was found in the small compartments, of which, 45 and 40% of AtGALT29A [Arabidopsis thaliana galactosyltransferase from family 29 (At1g08280)] and AtGlcAT14A [Arabidopsis thaliana glucuronosyltransferase from family 14 (At5g39990)] colocalized with AtGALT31A, respectively; in contrast, N-glycosylation enzymes rarely colocalized (3-18%), implicating a role of the small compartments in a part of arabinogalactan (O-glycan) biosynthesis rather than N-glycan processing. The dual localization of AtGALT31A was also observed for fluorescently tagged AtGALT31A stably expressed in an Arabidopsis atgalt31a mutant background. Further, site-directed mutagenesis of a phosphorylation site of AtGALT29A (Y144) increased the frequency of the protein being targeted to the AtGALT31A-localized small compartments, suggesting a role of Y144 in subcellular targeting. The AtGALT31A localized to the small compartments were colocalized with neither SYP61 (syntaxin of plants 61), a marker for trans-Golgi network (TGN), nor FM4-64-stained endosomes. However, 41% colocalized with EXO70E2 (Arabidopsis thaliana exocyst protein Exo70 homolog 2), a marker for exocyst-positive organelles, and least affected by Brefeldin A and Wortmannin. Taken together, AtGALT31A localized to small compartments that are distinct from the Golgi apparatus, the SYP61-localized TGN, FM4-64-stained endosomes and Wortmannin-vacuolated prevacuolar compartments, but may be part of an unconventional protein secretory pathway represented by EXO70E2 in plants.
authors: Department of Plant and Environmental Sciences, Faculty of Science, University of Copenhagen, Thorvaldsensvej 40, Frederiksberg C, 1871, Denmark.
authors: Christian Peter
authors: CP
authors: Poulsen
authors: Adiphol
authors: A
authors: Dilokpimol
authors: Grégory
authors: G
authors: Mouille
authors: Meike
authors: M
authors: Burow
authors: Naomi
authors: N
authors: Geshi
mesh_terms: D017360:Arabidopsis / Q000201:enzymology / Q000378:metabolism; D011992:Endosomes / Q000378:metabolism; D005685:Galactans / Q000378:metabolism*; D016695:Glycosyltransferases / Q000235:genetics / Q000378:metabolism*; D020125:Mutation, Missense; D010940:Plant Proteins / Q000235:genetics / Q000378:metabolism*; D055571:Secretory Pathway; D014026:Nicotiana / Q000201:enzymology / Q000378:metabolism; D021601:trans-Golgi Network / Q000378:metabolism
chemical_list: D005685:Galactans; D010940:Plant Proteins; D016695:Glycosyltransferases; C005653:arabinogalactan
keywords: Golgi apparatus; arabinogalactan proteins; exocyst-positive organelle; glycosyltransferase; plant cell walls; protein O-glycosylation; proteoglycan biosynthesis; subcellular localization; type II arabinogalactan; unconventional secretory pathway