In all land plants, cellulose is synthesized from hexameric plasma membrane complexes. Indirect evidence suggests that in vascular plants the complexes involved in primary wall synthesis contain three distinct cellulose synthase catalytic subunits (CESAs). In this study, we show that CESA3 and CESA6 fused to GFP are expressed in the same cells and at the same time in the hypocotyl of etiolated seedlings and migrate with comparable velocities along linear trajectories at the cell surface. We also show that CESA3 and CESA6 can be coimmunoprecipitated from detergent-solubilized extracts, their protein levels decrease in mutants for either CESA3, CESA6, or CESA1 and CESA3, CESA6 and also CESA1 can physically interact in vivo as shown by bimolecular fluorescence complementation. We also demonstrate that CESA6-related CESA5 and CESA2 are partially, but not completely, redundant with CESA6 and most likely compete with CESA6 for the same position in the cellulose synthesis complex. Using promoter-␤-glucuronidase fusions we show that CESA5, CESA6, and CESA2 have distinct overlapping expression patterns in hypocotyl and root corresponding to different stages of cellular development. Together, these data provide evidence for the existence of binding sites for three distinct CESA subunits in primary wall cellulose synthase complexes, with two positions being invariably occupied by CESA1 and CESA3, whereas at least three isoforms compete for the third position. Participation of the latter three isoforms might fine-tune the CESA complexes for the deposition of microfibrils at distinct cellular growth stages. C ellulose microfibrils are synthesized from a multiprotein complex inserted into the plasma membrane. These ''rosette'' complexes consist of six globules, each of which contains multiple cellulose synthase catalytic subunits (CESAs). These complexes migrate in the plasma membrane along microtubules, propelled by the polymerization of the ␤-1,4-glucan chains (1).Plant CESA genes are members of multigene families. Arabidopsis has 10 CESA isoforms that, based on sequence comparison with other plant species, can be classified into six orthologous groups (2). Mutational analysis shows that these six groups of isoforms have nonredundant functions in cellulose synthesis. Mutants for three isoforms (CESA4, CESA7, and CESA8) show defects in cellulose synthesis specifically in secondary walls (3)(4)(5). Microarray data show that the mRNAs for the three genes are coregulated (6, 7). The three proteins are expressed in the same cell types during secondary cell wall deposition, and co-immunoprecipitation (IP) experiments show that all three proteins interact (3). Although the interactions remain to be validated in vivo, these data strongly suggest that at least in these cells the complexes contain three isoforms. Mutants for isoforms CESA1, CESA3, and CESA6 have cellulose defects in primary cell walls (8-11). The three genes are also coregulated at the mRNA level (12). It is not known, however, whether the corresponding proteins are ...

Plant growth and organ formation depend on the oriented deposition of load-bearing cellulose microfibrils in the cell wall. Cellulose is synthesized by plasma membrane-bound complexes containing cellulose synthase proteins (CESAs). Here, we establish a role for the cytoskeleton in intracellular trafficking of cellulose synthase complexes (CSCs) through the in vivo study of the green fluorescent protein (GFP)-CESA3 fusion protein in Arabidopsis thaliana hypocotyls. GFP-CESA3 localizes to the plasma membrane, Golgi apparatus, a compartment identified by the VHA-a1 marker, and, surprisingly, a novel microtubule-associated cellulose synthase compartment (MASC) whose formation and movement depend on the dynamic cortical microtubule array. Osmotic stress or treatment with the cellulose synthesis inhibitor CGA 325'615 induces internalization of CSCs in MASCs, mimicking the intracellular distribution of CSCs in nongrowing cells. Our results indicate that cellulose synthesis is coordinated with growth status and regulated in part through CSC internalization. We find that CSC insertion in the plasma membrane is regulated by pauses of the Golgi apparatus along cortical microtubules. Our data support a model in which cortical microtubules not only guide the trajectories of CSCs in the plasma membrane, but also regulate the insertion and internalization of CSCs, thus allowing dynamic remodeling of CSC secretion during cell expansion and differentiation.

The mechanism of NEMO recruitment into supramolecular complexes and its dependence on ubiquitination differs in response to the proinflammatory cytokines TNF and IL-1.

Plant growth and organ formation depend on the oriented deposition of load-bearing cellulose microfibrils in the cell wall. Cellulose is synthesized by a large relative molecular weight cellulose synthase complex (CSC), which comprises at least three distinct cellulose synthases. Cellulose synthesis in plants or bacteria also requires the activity of an endo-1,4-b-D-glucanase, the exact function of which in the synthesis process is not known. Here, we show, to our knowledge for the first time, that a leaky mutation in the Arabidopsis (Arabidopsis thaliana) membrane-bound endo-1,4-b-D-glucanase KORRIGAN1 (KOR1) not only caused reduced CSC movement in the plasma membrane but also a reduced cellulose synthesis inhibitor-induced accumulation of CSCs in intracellular compartments. This suggests a role for KOR1 both in the synthesis of cellulose microfibrils and in the intracellular trafficking of CSCs. Next, we used a multidisciplinary approach, including live cell imaging, gel filtration chromatography analysis, split ubiquitin assays in yeast (Saccharomyces cerevisiae NMY51), and bimolecular fluorescence complementation, to show that, in contrast to previous observations, KOR1 is an integral part of the primary cell wall CSC in the plasma membrane.