The β(1 → 4)-linked polysaccharide synthase gene family in cereals: Towards identifying the mixed -linkage (1 → 3),(1 → 4)β-D-glucan synthase

Claudia Elena Vergara C., Purdue University

Abstract

A unique characteristic of the primary cell wall of grasses is that it synthesizes mixed linkage (1 → 3),(1 → 4)β-D-glucan (β-glucan) during cell expansion. The synthesis of β-glucan occurs at the Golgi apparatus. Biochemical approaches to identify polypeptides associated with the synthase complex led to the identification of three major intrinsic Golgi-membrane polypeptides, possibly associated with secresion and targeting of vesicle contents. Also, sucrose synthase (SuSy) is detected immunocytochermically in Golgi membrane fractions. This association might control the substrate supply to the active site of the synthase. Identification of a potential β-glucan synthase proved intractable by these methods. Therefore, I explored molecular approaches based on the availability of sequences encoding putative cellulose synthases (CesAs). The CesA genes encoded proteins contain eight membrane-spanning domains and four “U-motifs” with conserved aspartate residues that are essential for catalysis, and a QxxRW motif involved in processivity. In higher plants, there are two plant-specific insertions, a conserved region and an apparently “hypervariable region” (HVR). Phylogenetic relationships in the CesA gene families from Oryza sativa and Zea mays with Arabidopsis thaliana and other dicotyledonous species, reveal distinct sub-classes among the CesA genes. The sub-class identity is primarily defined by the HVR; the sequence in this region is highly conserved within sub-classes. Therefore, I propose that the region is more aptly termed a “Class-Specific Region” (CSR). Whereas some sub-classes are populated by CesAs from all species, two sub-classes are populated solely by CesAs from grass species. I suggest that one of these sub-classes, unique to cereals, may include β-glucan synthase genes. Conservation of particular motifs within the CSR suggests that this region may function in the catalytic reaction. Similar motifs are conserved in processive glycosyl transferases involved in the synthesis of non-cellulosic polymers with (1 → 4)β-linked backbones, including chitin, heparan sulfate, and hyaluronan. These analyses indicate that some of the CesA gene sub-classes may encode non-cellulosic (1 → 4)β-glycan synthases as well as cellulose synthases.

Degree

Ph.D.

Advisors

Carpita, Purdue University.

Subject Area

Molecular biology|Plant sciences

COinS