beta-D-Glycan synthases and the CesA gene family: lessons to be learned from the mixed-linkage (1 -> 3),(1 -> 4)beta-D-glucan synthase

Cellulose synthase genes (CesAs) encode a broad range of processive glycosy ltransferases that synthesize (1 -->4)beta -D-glycosyl units. The proteins predicted to be encoded by these genes contain up to eight membrane-spannin g domains and four 'U-motifs' with conserved aspartate residues and a QxxRW motif that are essential for substrate binding and catalysis. In higher pl ants, the domain structure includes two plant-specific regions, one that is relatively conserved and a second, so-called 'hypervariable region' (HVR). Analysis of the phylogenetic relationships among members of the CesA multi -gene families from two grass species, Oryza sativa and Zea mays, with Arab idopsis thaliana and other dicotyledonous species reveals that the CesA gen es cluster into several distinct sub-classes. Whereas some sub-classes are populated by CesAs from all species, two sub-classes are populated solely b y CesAs from grass species. The sub-class identity is primarily defined by the HVR, and the sequence in this region does not vary substantially among members of the same sub-class. Hence, we suggest that the region is more ap tly termed a 'class-specific region' (CSR). Several motifs containing cyste ine, basic, acidic and aromatic residues indicate that the CSR may function in substrate binding specificity and catalysis. Similar motifs are conserv ed in bacterial cellulose synthases, the Dictyostelium discoideum cellulose synthase, and other processive glycosyltransferases involved in the synthe sis of non-cellulosic polymers with (1 -->4)beta -linked backbones, includi ng chitin, heparan, and hyaluronan. These analyses re-open the question whe ther all the CesA genes encode cellulose synthases or whether some of the s ub-class members may encode other non-cellulosic (1 -->4)beta -glycan synth ases in plants. For example, the mixed-linkage (1 -->3)(1 -->4)beta -D-gluc an synthase is found specifically in grasses and possesses many features mo re similar to those of cellulose synthase than to those of other beta -link ed cross-linking glycans. In this respect, the enzymatic properties of the mixed-linkage beta -glucan synthases not only provide special insight into the mechanisms of (1 -->4)beta -glycan synthesis but may also uncover the g enes that encode the synthases themselves.