Primary Source: Lei, L., et al., CELLULOSE SYNTHASE INTERACTIVE1 Is Required for Fast Recycling of Cellulose Synthase Complexes to the Plasma Membrane in Arabidopsis. Plant Cell, 2015. 27(10): p. 2926-2940.
Cellulose is a vital component of plant cell walls that confers physical strength and biological resilience to the plant. Within plant tissue cellulose occurs in both amorphous and highly crystalline forms that are both produced by the processive addition of UDP–glucose molecules to long homo-polymeric chains. Polymerization is catalyzed by large multi-enzyme cellulose synthase complexes (CSCs) that span the plasma membrane. Intracellular domains catalyze the addition of each UDP–glucose unit which forces the growing cellulose chain through a membrane channel formed by interactions among transmembrane domains and into the extracellular space where crystallization and/or interactions with other cell wall polymers (hemicellulose, lignin, etc.) can occur. Cellulose ‘extrusion’ is believed to drive the observed translational mobility of actively polymerizing CSCs within the plane of the cell membrane. In addition, this translational motion is guided by linking CSCs to the apical microtubules lying just below the plasma membrane with an adaptor protein known as CELLULOSE SYNTHASE INTERACTIVE1 (CSI1). The microtubule-to-CSC coupling ensures that the synthesized cellulose is drawn into elongated chains that are favorable for forming cell wall structures. This microtubule–CSI1–CSC system is functionally analogous to contemporary 3D printing in which a head (CSCs) extrude a structural polymeric material (cellulose) guided by a track system (CSI1 and apical microtubules). Plants were 3D printing before it was cool! The figure below shows schematically the components of cellulose biosynthesis.
Figure Source: Bashline, L., S.D. Li, and Y. Gu, The trafficking of the cellulose synthase complex in higher plants. Annals of Botany, 2014. 114(6): p. 1059-1067. ©The Authors. Reprinted with permission.
While cellulose synthesis is an elegant biological process, the “on/off” switch for polymerization is a bit, well, clunky. CSCs synthesize cellulose whenever they are located at the plasma membrane and require translocation from the membrane to stop the synthesis. Often when transmembrane proteins are removed from the membrane the protein is targeted to a degradation or turnover process, and new protein chains must be synthesized to replace the degraded protein. In the case of the CSC, turnover of the complex would be an expensive way to achieve control of cellulose synthesis and create lag in the cell’s ability to resume cellulose synthesis under favorable conditions. Lei et al. have recently shown that the connection between CSCs and apical microtubules formed by CSI1 plays an important role in a membrane translocation process by which functional CSCs can be removed from the membrane but not targeted for turnover. CSCs that are removed from the plasma membrane form protein–lipid vesicles known as small microtubule-associated cellulose synthase complexes (SMaCCs). By examining CSI1 and cellulose synthase cellular localization in Arabidopsis thaliana plants expressing different CSI1 domain deletion mutants, Lei et al. revealed that CSI is vital to the formation of SMaCCs and that SMaCCs localize with apical microtubules just beneath the plasma membrane by a CSI1-dependent interaction. Furthermore, the authors demonstrated by watching the recovery of CSCs in the plasma membrane that the ability to form SMaCCs speeds the recovery of cellulose synthesis after release from polymerization-inhibiting stress. SMaCCs enabled by CSI1 act like little recycling bins that instead of allowing CSCs to go to the “shredder” of the turnover machinery keep the CSCs safe and poised to be returned to use for cellulose synthesis at the plasma membrane. Plants were also recycling before it was cool!
Perhaps nature should inspire us with respect to 3D printing and to recycling and sustainability?