The Cellulose Synthase Protein

The cellulose synthase genes identified in A. xylinum encode either the catalytic subunit consisting of 754 amino acids and 9 potential transmembrane regions or a longer protein of approximately 1,550 amino acids consisting of the cellulose synthase catalytic domain and an activator (c-di-GMP)-binding domain with 9 potential transmembrane regions (Saxena et al., 1990, 1991, 1994; Wong et al., 1990). The catalytic region in these proteins was predicted to have the conserved residues and sequence motif identified as D, D, D, QXXRW (Saxena et al., 1995). CesA genes in plants encode a large, multipass transmembrane protein with a globular region containing the D, D, D, QXXRW motif. The CesA proteins in plants have a plant-specific conserved region (CR-P) and a hypervariable region (HVR-2) within the cytosolic globular region that contains the conserved residues. A conserved, extended N-terminal region is shown to have two zinc-finger domains resembling LIM/RING domains followed by a HVR-1 region (Kawagoe and Delmer, 1997). The RING domains are predicted to mediate protein–protein interactions. Using the yeast two-hybrid system, it has been shown that the zinc-finger domain of GhCesA1 is able to interact with itself to form homodimers or heterodimers with the zinc-finger domain of GhCesA2 in a redox-dependent manner (Kurek et al., 2002). This dimerization of CesAs is supposed to represent the first stage in the assembly of the rosette TC (Saxena and Brown, 2005).