Xyloglucan xylosyltransferase stem region mediates heterodimer formation.

Xyloglucans are plant cell wall hemicelluloses composed of a glucan backbone substituted with xylosyl residues with diverse branching patterns. Xyloglucan xylosyltransferases (XXT) 2 and XXT5 are both required to fully xylosylate the glucan backbone and physically interact, yet the mechanism of this interaction has remained undefined. Here, we present crystal structures of the XXT2-XXT5 heterodimer, organized as an obligatory heterodimer and bound to both substrates. These structures confirm the presence of a secondary binding pocket in XXT5 that likely accommodates a pre-xylosylated glucan chain. A previously uncharacterized interaction is also revealed between XXT2's stem region and XXT5, which is absent in the XXT1 homodimer structure. Truncated and chimeric protein experiments support that this stem polypeptide controls oligomerization, a mechanism likely shared by other glycosyltransferase complexes. Together, these findings provide the structural basis for understanding how XXTs coordinate xyloglucan xylosylation and offer insight into the mechanism of their dimerization.