Abstract
Carbohydrate-active enzyme glycosyltransferase family 8 (GT8) includes the plant galacturonosyltransferase1-related gene family of proven and putative alpha-galacturonosyltransferase (GAUT) and GAUT-like (GATL) genes. We computationally identified and investigated this family in 15 fully sequenced plant and green algal genomes and in the National Center for Biotechnology Information nonredundant protein database to determine the phylogenetic relatedness of the GAUTs and GATLs to other GT8 family members. The GT8 proteins fall into three well-delineated major classes. In addition to GAUTs and GATLs, known or predicted to be involved in plant cell wall biosynthesis, class I also includes a lower plant-specific GAUT and GATL-related (GATR) subfamily, two metazoan subfamilies, and proteins from other eukaryotes and cyanobacteria. Class II includes galactinol synthases and plant glycogenin-like starch initiation proteins that are not known to be directly involved in cell wall synthesis, as well as proteins from fungi, metazoans, viruses, and bacteria. Class III consists almost entirely of bacterial proteins that are lipooligo/polysaccharide alpha-galactosyltransferases and alpha-glucosyltransferases. Sequence motifs conserved across all GT8 subfamilies and those specific to plant cell wall-related GT8 subfamilies were identified and mapped onto a predicted GAUT1 protein structure. The tertiary structure prediction identified sequence motifs likely to represent key amino acids involved in catalysis, substrate binding, protein-protein interactions, and structural elements required for GAUT1 function. The results show that the GAUTs, GATLs, and GATRs have a different evolutionary origin than other plant GT8 genes, were likely acquired from an ancient cyanobacterium (Synechococcus) progenitor, and separate into unique subclades that may indicate functional specialization.
Highlights
Plant cell walls are composed of three principal types of polysaccharides: cellulose, hemicellulose and pectin
If two sequences of varying length, but 100% identical over the shorter length, were identified, only the longer one was kept. Another 1,708 glycosyltransferase family 8 (GT8) domains in 1,701 proteins were identified in the NCBI-nr database, and termed the nr-GT8 set
Together with our findings reported here, this suggests that the key progenitor enzymes responsible for the biosynthesis of the backbones of all three of the major classes of plant cell wall polysaccharides were in existence anciently in the bacterial world, and that at least some of them were subsequently transferred into plants, possibly at, or after the time of, establishment of plastids in the ancestral plant
Summary
Plant cell walls are composed of three principal types of polysaccharides: cellulose, hemicellulose and pectin. Studying the biosynthesis and degradation of these biopolymers is important because cell walls have multiple roles in plants, including providing structural support to cells and defense against pathogens, serving as cell-specific developmental and differentiation markers, and mediating or facilitating cell-cell communication In addition to their important roles within plants, cell walls have many economic uses in human and animal nutrition, and as sources of natural textile fibers, paper and wood products and as components of fine chemicals and medicinal products. Analyses of fully sequenced plant genomes have revealed that they encode hundreds or even thousands of carbohydrate active enzymes (Henrissat et al, 2001; Yokoyama and Nishitani, 2004; Geisler-Lee et al, 2006) Most of these carbohydrate active enzymes (CAZy) (Cantarel et al, 2009) are glycosyltransferases (GTs) or glycoside hydrolases (GHs), which are key players in plant cell wall biosynthesis and modification (Cosgrove, 2005)
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