A mutation in the catalytic domain of cellulose synthase 6 halts its transport to the Golgi apparatus

Cellulose microfibrils, which form the mechanical framework of the plant cell wall, are synthesized by the cellulose synthase complex in the plasma membrane. Here, we introduced point mutations into the catalytic domain of cellulose synthase 6 (CESA6) in Arabidopsis to produce enhanced yellow fluorescent protein (EYFP)-tagged CESA6(D395N), CESA6(Q823E), and CESA6(D395N+Q823E), which were exogenously produced in a cesa6 null mutant, prc1-1. Comparison of these mutants in terms of plant phenotype, cellulose content, cellulose synthase complex dynamics, and organization of cellulose microfibrils showed that prc1-1 expressing EYFP:CESA6(D395N) or CESA6(D395N+Q823E) was nearly the same as prc1-1, whereas prc1-1 expressing EYFP:CESA6(Q823E) was almost identical to wild type and prc1-1 expressing EYFP:WT CESA6, indicating that CESA6(D395N) and CESA6(D395N+Q823E) do not function in cellulose synthesis, while CESA6(Q823E) is still functionally active. Total internal reflection fluorescence microscopy and confocal microscopy were used to monitor the subcellular localization of these proteins. We found that EYFP:CESA6(D395N) and EYFP:CESA6(D395N+Q823E) were absent from subcellular regions containing the Golgi and the plasma membrane, and they appeared to be retained in the endoplasmic reticulum. By contrast, EYFP:CESA6(Q823E) had a normal localization pattern, like that of wild-type EYFP:CESA6. Our results demonstrate that the D395N mutation in CESA6 interrupts its normal transport to the Golgi and its eventual participation in cellulose synthase complex assembly.