Interaction with COPII Member SAR1 Is Critical for the Delivery of Arabidopsis Xyloglucan Xylosyltransferases XXT2 and XXT5 to the Golgi Apparatus

Transport of Golgi-localized proteins from the ER is mediated by the coat protein complex II (COPII) and its members, COPII inner coat subunit Sec24 and Secretion-associated Ras-related GTPase 1 (Sar1). Sar1 and Sec24 recognize cytosolic N-termini of glycosyltransferases (GTs) that contain peptide signals required for incorporation into COPII-coated vesicles. Xyloglucan Xylosyltransferases (XXTs) are required for xyloglucan (XyGs) biosynthesis and must be transported to the Golgi for proper function. In this study, we demonstrated that XXTs interact with AtSar1 in the COPII complex but not with AtSec24, which was previously reported to be the main recruiter of cargo proteins into COPII-coated vesicles. The mutation of the arginine to glutamine residues of di-arginine motifs in the N-termini of XXTs caused protein mislocalization and significantly reduced the strength of the interaction with AtSar1. These mutations caused 90% of XXTs to either remain in the ER or localize to small non-Golgi compartments. In turn, such mislocalization significantly suppressed the recovery of XyGs biosynthesis in Arabidopsis thaliana (Arabidopsis) mutants (xxt1xxt2 and xxt3xxt4xxt5), failing to restore their root phenotypes to normal. Our results demonstrate the interaction between cargo and AtSar1, highlighting the critical role of di-arginine motifs in this interaction. These results provide new insights into the mechanism of ER-to-Golgi delivery of plant GTs, which significantly advances our understanding of polysaccharide biosynthesis in the Golgi and the enzymes responsible for it.