A class of plant glycosyltransferases involved in cellular homeostasis

Abstract

Many small lipophilic compounds in living cells can be modified by glycosylation. These processes can regulate the bioactivity of the compounds, their intracellular location and their metabolism. The glycosyltransferases involved in biotransformations of small molecules have been grouped into Family 1 of the 69 families that are classified on the basis of substrate recognition and sequence relatedness. In plants, these transfer reactions generally use UDP-glucose with acceptors that include hormones such as auxins and cytokinins, secondary metabolites such as flavonoids, and foreign compounds including herbicides and pesticides. In mammalian organisms, UDP-glucuronic acid is typically used in the transfer reactions to endogenous acceptors, such as steroid and thyroid hormones, bile acids and retinoids, and to xenobiotics, including nonsteroidal anti-inflammatory drugs and dietary metabolites. There is widespread interest in this class of enzyme since they are known to function both in the regulation of cellular homeostasis and in detoxification pathways. This review outlines current knowledge of these glycosyltransferases drawing on information gained from studies of plant and mammalian enzymes.

Figure 1

Family 1 constitute 7% of all glycosyltransferases that have been classified to date. This review focuses on the 48% of Family 1 containing the consensus sequence. The statistical data of Family 1 glycosyltransferases with and without the consensus are provided by Dr Pedro Coutinho and Professor Bernard Henrissat (AFMB-CNRS, France). The reader is referred to the CAZy (carbohydrate-active enzymes) website (http://afmb.cnrs-mrs.fr/CAZY/) for the up to date information. The 44-amino-acid consensus sequence defining the class of Family 1 glycosyltransferases described in this review is illustrated.

Figure 2

(A) Ribbon diagrams of two representative GT structures. A. orientalis GtfA (Family 1, PBD accession code 1PNV) and Bacillus subtilis SpsA (Family 2, PBD accession code 1H7L) were selected to illustrate the GT-B and GT-A structures, respectively. (B) Arabidopsis UGT71C1 and human UGT1A1 were chosen as the representative glycosyltransferases containing the consensus. Their secondary structures were predicted using a web-based programme (http://cubic.bioc.columbia.edu/predictprotein/) and were compared to the secondary structure of GtfA (without the consensus) gained from the crystallographic study.

Figure 3

Phenotype of a transgenic line constitutively overexpressing an Arabidopsis gene UGT84B1 encoding a glycosyltransferase of the auxin indole-3-acetic acid. The aerial tissues of the transgenic Arabidopsis plant (upper panel) showed a higher degree of branching and shorter stature compared to wild type. When seedlings (lower panel) were grown vertically, the transgenic root system displayed a phenotype of impaired gravitropism.

Figure 4

(A) The sequences illustrated are those of the phylogenetic group E of Arabidopsis glycosyltransferases (Li et al, 2001). Regioselective glycosylation of esculetin falls into two distinct subsets in Group E, with the exception of UGTs 72B1 and 72B3, which suggests that a switching event in regioselectivity has occurred during evolution (Lim et al, 2003a). Sequences not analysed are labelled in black. (B) UGTs 74F1 and 74F2 are 82% identical at the amino-acid sequence level, but display different regioselectivity towards salicylic acid. (C) UrdGT1b and UrdGT1c are Streptomyces fradiae glycosyltransferases involved in the biosynthesis of the antibiotic urdamycin. Gene shuffling using the DNA sequences encoding these two glycosyltransferases enabled the generation of an engineered protein (cyan) with a catalytic activity different from the parental enzymes (Hoffmeister et al, 2002).