GDP-L-fucose transport in plants: The missing piece

The monosaccharide fucose is an important deoxyhexose occurring in plants, fungi and animals. Fucose is mainly found as a component of glycan structures such as Nand O-linked glycans, glycolipids and in cell wall polysaccharides such as fucoidan, xyloglucan, rhamnogalacturonan II. The biosynthesis of these glycan structures is mediated by glycosyltransferases and typically occurs within the endomembrane system. Fucosylation reactions require an activated sugar in the form of GDP-L-fucose (GDP-Fuc) which is either biosynthesized de novo or can also be salvaged from fucose in the cytosol. This partitioning necessitates active GDP-Fuc transport into the endomembrane system for glycosylation reactions. To date, GDP-Fuc transporters have been characterized in mammals (Homo sapiens), insects (Drosophila melanogaster) and nematodes (Caenorhabditis elegans), reviewed in. However, given the importance of fucose containing structures in plants, the absence of a characterized GDP-Fuc transporter has, until recently, been a dilemma. Thus, the recent identification of a GDP-Fuc transporter (GFT1) in the reference plant Arabidopsis thaliana represents the final missing piece of GDP-Fuc metabolism in plants. Considering the identification of a functional endomembrane GDP-Fuc transporter occurred over 15 y ago, it is intriguing that it has taken this long for the plant ortholog to be identified. This fact is even more surprising when we consider that the plant GDP-sugar transporter clade was elucidated in the same year with the identification of GONST1, the Arabidopsis GDP-mannose (GDP-Man) transporter. While in the intervening period members of the plant GDP-sugar (GONST) clade have been assessed for their capacity to transport GDP-Fuc, no candidate was identified. There are a number of reasons for this lack of success, but 2 main issues are likely responsible. First, nucleotide sugar transporters are antiporters and thus require an exchange substrate, and in the case of GDP-Fuc, there is a requirement for GMP. Unfortunately, the majority of transport assays undertaken with the nucleotide sugar transporter family do not use pre-loaded vesicles and assume that endogenous substrates (e.g. UMP or GMP) are present in excess and will drive transport in vitro. Second, the identification of candidate plant GDP-Fuc transporters using bioinformatic approaches is likely problematic; it is now clear that plant GDP-Fuc members (clade II) form a distinct clade when compared to their experimentally determined orthologues in animals (clade III, Fig. 1). This is in contrast to the GDP-Man clade where both the plant and fungal orthologues appear to form a tighter group (clade I). The recent identification of a fucose salvage pathway in the fungus Mortierella alpina as well as the identification of GDP-Fuc in metabolic extracts, supports the likelihood that a GDP-Fuc transporter is present in some fungal species. This observation could be readily confirmed with the addition of the top BLAST match for Mortierella sp. and Scleroderma sp to the phylogenetic tree and their subsequent homology to the characterized animal GDP-Fuc transporters