Higher plant cellulose synthases

Abstract

Cellulose, an aggregate of unbranched polymers of beta-1,4-linked glucose residues, is the major component of wood and thus paper, and is synthesized by plants, most algae, some bacteria and fungi, and even some animals. The genes that synthesize cellulose in higher plants differ greatly from the well-characterized genes found in Acetobacter and Agrobacterium sp. More correctly designated as 'cellulose synthase catalytic subunits', plant cellulose synthase (CesA) proteins are integral membrane proteins, approximately 1,000 amino acids in length. The sequences for more than 20 full-length CesA genes are available, and they show high similarity to one another across the entire length of the encoded protein, except for two small regions of variability. There are a number of highly conserved residues, including several motifs shown to be necessary for processive glycosyltransferase activity. No crystal structure is known for cellulose synthase proteins, and the exact enzymatic mechanism is unknown. There are a number of mutations in cellulose synthase genes in the model organism Arabidopsis thaliana. Some of these mutants show altered morphology due to the lack of a properly developed primary or secondary cell wall. Others show resistance to well-characterized cellulose biosynthesis inhibitors.

Figure 1

Gene structure of the Arabidopsis CesA gene family and the rice CesA7 gene, the only CesA genes for which full genomic sequence is available. At, Arabidopsis thaliana; Os, Oryza sativa. Exons are represented by boxes and introns by connecting lines. Exons or portions of exons encoding the domains shown in Figure 3 are colored as indicated.

Figure 2

A cladogram of the plant CesA superfamily and related non-plant proteins. ClustalX (version 1.8) was used to create an alignment of the protein sequences that was then bootstrapped (n = 1000 trials) to create the final tree. Subfamilies are indicated with colored bars on the right. At, Arabidopsis thaliana (thale cress); Gh, Gossypium hirsutum (cotton); Le, Lycopersicon esculentum (tomato); Mt, Medicago truncatula (barrel medic); Os, Oryza sativa (rice); Pt, Populus tremuloides (quaking aspen); Pt/Pa, Populus alba x Populus tremula (gray poplar); Zm, Zea mays (maize).

Figure 3

Protein features characteristic of plant cellulose synthase proteins, shown using the Arabidopsis CesA1 protein as a paradigm. Regions indicated above and below are described within the text and domains are colored as indicated.

Figure 4

Hypothetical three-dimensional structure of CesA proteins. The transmembrane helices of the CesA protein are thought to form a pore in the plasma membrane through which the growing glucan chain passes. Regions are colored to follow those shown in Figure 2. This figure is adapted from [6].