Plant Physiol. Drug Metab Dispos
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Published on September 3, 2008; 10.1104/pp.108.127027

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Received July 25, 2008
Accepted August 24, 2008

Comparative Analyses of Arabidopsis cgl1 (complex glycan 1) Mutants and Genetic Interaction with stt3a (staurosporin & temperature-sensitive 3a)

Julia Frank , Heidi Kaulfurst-Soboll , Stephan Rips , Hisashi Koiwa , and Antje von Schaewen *

Institut fur Botanik, Westfalische Wilhelms-Universitat Munster, Schlossgarten 3, 48149 Munster, Germany; Vegetable and Fruit Improvement Center, Department of Horticultural Sciences, and Molecular and Environmental Plant Science Program, Texas A&M University, College Station, Texas 77843-2133, USA

* Corresponding author; email: Schaewen{at}uni-muenster.de.

We compare three Arabidopsis thaliana cgl1 alleles and report on genetic interaction with stt3a. STT3a encodes a subunit of oligo-saccharyl-trans-ferase that affects efficiency of N-glycan transfer to nascent secretory proteins in the ER; cgl1 mutants lack N-acetyl-gluco-saminyl-trans-ferase-I activity and are unable to form complex N-glycans in the Golgi apparatus. By studying CGL1-GFP fusions in transient assays we show that the extra N-glyco-sy-la-tion site created by a point mutation in cgl1 C5 is used in planta and interferes with folding of full-length membrane-anchored polypeptides in the ER. Tunicamycin-treatment or expression in the stt3a-2 mutant relieved the folding block and migration to Golgi stacks resumed. Comple-men-ta-tion tests with C5-GFP and other N-glycosylation variants of CGL1 demonstrated that suppression of aberrant N-glyco-sylation restores activity. Interestingly, CGL1 seems to be functional also as non-glyco-----sy-la-ted enzyme. Two other cgl1 alleles showed splicing defects of their transcripts: In cgl1 C6, a point mutation affects the 3'-splice site of intron 14, resulting in frame shifts; and in cgl1-T, intron 11 fails to splice due to insertion of a T-DNA copy. Introgression of stt3a-2 did not restore complex glycan formation in cgl1 C6 or cgl1-T but suppressed the GnTI defect in cgl1 C5. Root growth assays revealed synergistic effects in double mutants cgl1 C6 stt3a-2 and cgl1-T stt3a-2 only. Besides demonstrating the condi-tional nature of cgl1 C5 in planta, our observations with loss-of-function alleles cgl1 C6 and cgl1-T in the stt3a-2 underglycosylation background prove that correct N-glyco-sy-la-tion is important for normal root growth and morphology in Arabidopsis.






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