N-Glycoproteomic Characterization of Mannosidase and Xylosyltransferase Mutant Strains of Chlamydomonas reinhardtii

At present, only little is known about the enzymatic machinery required for N -glycosylation in Chlamydomonas reinhardtii , leading to the formation of N -glycans harboring Xyl and methylated Man. This machinery possesses new enzymatic features, as C. reinhardtii N -glycans are independent of β1,2- N -acetylglucosaminyltransferase I. Here we have performed comparative N -glycoproteomic analyses of insertional mutants of mannosidase 1A (IM Man1A ) and xylosyltransferase 1A (IM XylT1A ). The disruption of man1A affected methylation of Man and the addition of terminal Xyl. The absence of XylT1A led to shorter N -glycans compared to the wild type. The use of a IM Man1A xIM XylT1A double mutant revealed that the absence of Man1A suppressed the IM XylT1A phenotype, indicating that the increased N -glycan trimming is regulated by core β1,2-Xyl and is dependent on Man1A activity. These data point toward an enzymatic cascade in the N -glycosylation pathway of C. reinhardtii with interlinked roles of Man1A and XylT1A. The results described herein represent the first step toward a functional characterization of the enzymatic N -glycosylation machinery in C. reinhardtii .