VvGONST-A and VvGONST-B are Golgi-localised GDP-sugartransporters in grapevine (Vitis vinifera L.)

Abstract

tPlant nucleotide-sugar transporters (NSTs) are responsible for the import of nucleotide-sugar substratesinto the Golgi lumen, for subsequent use in glycosylation reactions. NSTs are specific for either GDP-or UDP-sugars, and almost all transporters studied to date have been isolated from Arabidopsis thalianaL. In order to determine the conservation of the import mechanism in other higher plant species, herewe report the identification and characterisation of VvGONST-A and VvGONST-B from grapevine (Vitisvinifera L. cv. Thompson Seedless), which are the orthologues of the GDP-sugar transporters GONST3and GONST4 in Arabidopsis. Both grapevine NSTs possess the molecular features characteristic of GDP-sugar transporters, including a GDP-binding domain (GXL/VNK) towards the C-terminal. VvGONST-A andVvGONST-B expression is highest at berry setting and decreases throughout berry development and ripen-ing. Moreover, we show using green fluorescent protein (GFP) tagged versions and brefeldin A treatments,that both are localised in the Golgi apparatus. Additionally, in vitro transport assays after expression ofboth NSTs in tobacco leaves indicate that VvGONST-A and VvGONST-B are capable of transporting GDP-mannose and GDP-glucose, respectively, but not a range of other UDP- and GDP-sugars. The possiblefunctions of these NSTs in glucomannan synthesis and/or glycosylation of sphingolipids are discussed