The interaction of the Arabidopsis Xyloglucan Xylosyltransferases XXTs with the COPII member SAR1 via their di-Arginine motifs is critical for delivery to the Golgi.

Golgi-localized Xyloglucan Xylosyltransferases (XXT2 and XXT5) participate in xyloglucan biosynthesis, and to do this, they require the proper localization. The COPII complex is responsible for delivering cargo proteins from the ER to the Golgi, which is facilitated by the complexs member proteins Sec24 and Sar1. Additionally, the N-termini of glycosyltransferases (GTs) play a crucial role in their transportation and localization. In this study, we demonstrated for the first time that XXTs interact with Sar1 protein in the COPII complex but not with Sec24, 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 XXT2 and XXT5 caused protein mislocalization and significantly reduced the strength of the interaction with Sar1. These mutations caused 90% of XXTs to either remain in the ER or localize to non-Golgi small compartments. In turn, such mislocalization significantly suppressed the recovery of xyloglucan biosynthesis in Arabidopsis thaliana (Arabidopsis) mutant plants (xxt1xxt2 and xxt3xxt4xxt5), failing to restore their root phenotypes to normal. Our results demonstrate the interaction between cargo proteins and Sar1 proteins, 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. Significance statementThis study demonstrates that plant glycosyltransferases directly interact with the SAR1 protein of the COPII complex. The di-arginine motifs present in the N-termini of glycosyltransferases play a critical role in cargo selection and transport from the ER to the Golgi apparatus via COPII-coated vesicles, while interacting with SAR1.