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Molecular cloning and characterization of Arabidopsis thaliana Golgi [alpha]-mannosidase II, a key enzyme in the formation of complex N-glycans in plants

Strasser, Richard, Schoberer, Jennifer, Jin, Chunsheng, Glössl, Josef, Mach, Lukas, Steinkellner, Herta
The plant journal 2006 v.45 no.5 pp. 789-803
Arabidopsis thaliana, Golgi apparatus, amino acid sequences, amino acids, complementary DNA, enzymes, hybrids, knockout mutants, membrane proteins, molecular cloning, post-translational modification
N-glycosylation is one of the major post-translational modifications of proteins in eukaryotes; however, the processing reactions of oligomannosidic N-glycan precursors leading to hybrid-type and finally complex-type N-glycans are not fully understood in plants. To investigate the role of Golgi [alpha]-mannosidase II (GMII) in the formation of complex N-glycans in plants, we identified a putative GMII from Arabidopsis thaliana (AtGMII; EC and characterized the enzyme at a molecular level. The putative AtGMII cDNA was cloned, and its deduced amino acid sequence revealed a typical type II membrane protein of 1173 amino acids. A soluble recombinant form of the enzyme produced in insect cells was capable of processing different physiologically relevant hybrid N-glycans. Furthermore, a detailed N-glycan analysis of two AtGMII knockout mutants revealed the predominant presence of unprocessed hybrid N-glycans. These results provide evidence that AtGMII plays a central role in the formation of complex N-glycans in plants. Furthermore, conclusive evidence was obtained that alternative routes in the conversion of hybrid N-glycans to complex N-glycans exist in plants. Transient expression of N-terminal AtGMII fragments fused to a GFP reporter molecule demonstrated that the transmembrane domain and 10 amino acids from the cytoplasmic tail are sufficient to retain a reporter molecule in the Golgi apparatus and that lumenal sequences are not involved in the retention mechanism. A GFP fusion construct containing only the transmembrane domain was predominantly retained in the ER, a result that indicates the presence of a motif promoting ER export within the last 10 amino acids of the cytoplasmic tail of AtGMII.