N-Glycosylation in the Moss Physcomitrella patens is Organized Similarly to that in Higher Plants

Abstract: Allergenicity of plant glycoproteins in humans may prevent the use of plants as production factories for pharmaceutically important proteins. The major difference between plant and mammalian N-glycans is the presence of xylosyl and α1,3-fucosyl residues in the former. In a first step towards "humanization" of the N-glycosylation pathway in the moss Physcomitrella patens, which could be an excellent system for industrial production of therapeutic proteins, we isolated the cDNAs and genes for N-acetylglucosaminyltransferase I (GNTI), α1,3-fucosyltransferase, and β1,2-xylosyltransferase. Sequence analysis revealed that all three proteins are homologous to their counterparts from higher plants, however, the conservation of the primary structure was only 35 - 45 %. The gene encoding the key enzyme of the pathway, gntI, was disrupted in P. patens by homologous recombination. Although the mutation of this gene in mouse or A. thaliana led to a significantly altered pattern of N-glycans, the glycosylation pattern in the gntI knockouts did not differ from that in wild-type moss and was identical to that in higher plants. Protein secretion, analysed in assays with recombinant human VEGF₁₂₁ protein, was not affected in the knockouts. We conclude from our findings that the N-glycosylation pathway in P. patens is identically organized to that in higher plants. However, P. patens probably possesses more than one isoform of GNTI which complicates a straightforward knockout. Therefore, and since complex type structures appear more desirable than oligomannosidic N-glycans, future modifications of the pathway should target α1,3-fucosyltransferase and/or β1,2-xylosyltransferase.