Plant cell walls contain cellulose fibres embedded in a pectin gel and several protein-carbohydrate polymer complexes. Previous work indicated that these complexes are assembled at the cell wall, but on Bruce Kohorn and co-authors report that the crosslinking of carbohydrate polymers to a protein called wall-associated kinase 1 (WAK1) begins in a cytoplasmic compartment in Arabidopsis. The authors investigated the biosynthesis of WAK1, which binds to pectin through an extracellular domain, by expressing a WAK1-GFP fusion protein in plant cells regenerating their cell walls. They found that WAK1-GFP accumulates in a cytoplasmic compartment that contains pectin and Golgi markers; WAK1 then migrates very slowly to the cell wall. The authors show that the migration requires cellulose synthesis but the crosslinking of WAK1 into a detergent-insoluble complex, which occurs in the cytoplasm, is independent of cellulose synthesis. The authors conclude that WAK1 crosslinking, which is likely to be essential for its roles in cell elongation and sugar metabolism, occurs deep within the cell rather than at the plasma membrane as previously thought.
