Expression patterns and subcellular localization of a 52 kDa sucrose-binding protein homologue of Vicia faba (VfSBPL) suggest different functions during development

A cDNA coding for a 54 kDa signal sequence containing protein has been isol ated from a faba bean cotyledonary library and characterized. The deduced p rotein is designated Vicia faba SBP-like protein (VfSBPL) since it shares 5 8% homology to a 62 kDa soybean (Glycine max) protein (GmSBP) which has bee n described as a sucrose-binding and sucrose-transporting protein (SBP). Vf SBPL as well as GmSBP are outgroup members of the large vicilin storage pro tein family. We were unable to measure any sucrose transport activity in mu tant yeast cells expressing VfSBPL. During seed maturation in late (stage V II) cotyledons mRNA was localized by in situ hybridization in the storage p arenchyma cells. At the subcellular level, immunolocalization studies prove d VfSBPL accumulation in storage protein vacuoles. However, mRNA localizati on in stage VI cotyledons during the pre-storage/storage transition phase w as untypical for a storage protein in that, in addition to storage parenchy ma cell labelling, strong labelling was found over seed coat vascular stran ds and the embryo epidermal transfer cell layer reminiscent of sucrose tran sporter localization. The VfSBPL gene is composed of 6 exons and 5 introns with introns located at the same sites as in a Vicia faba 50 kDa vicilin st orage protein gene. The time pattern of expression as revealed by northern blotting and the GUS accumulation pattern caused by a VfSBPL-promoter/GUS c onstruct in transgenic tobacco seeds was similar to a seed protein gene wit h increasing expression during seed maturation. Our data suggest different functions of VfSBPL during seed development.