Functional and physiological characterization of the Arabidopsis monosaccharide-transporter-like proteins At1g05030, At1g67300 and At1g79820 from endomembranes

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Arabidopsis has a large family (more than 50 members) of monosaccharide transporters (MST) and MST-like (MSTL) transporters that fall into 7 clearly distinguishable subgroups. All of these proteins are predicted to have 12 transmembrane (TM) helices. More than 50% of these proteins were characterized in our group and shown to catalyze the transport of different hexoses, or linear or cyclic polyols across the plasma membranes or across the tonoplast of different cell types. One of these seven subgroups with only four proteins contains one protein that localizes to plastids (At5g16150; AtpGlcT) and one protein that localizes to the Golgi (At1g79820; SGB1). In the proposed project we plan to study the two remaining, yet uncharacterized proteins of this subgroup, At1g05030 and At1g67300. In preliminary analyses we could show that these proteins are Golgi- (At1g05030) or tonoplast-localized (At1g67300). Moreover, we could demonstrate the existence of shorter, alternatively spliced mRNAs for At1g67300 and for the already published Golgi-localized transporter At1g79820 (SGB1). These shorter mRNAs encode proteins with only 6 TM-helices that were not observed for any of the previously studied proteins of the MSTL superfamily. We aim to functionally and physiologically characterize the proteins encoded by the At1g05030 and At1g67300 (12 TM) genes and to elucidate the role of the proteins encoded by the shorter transcripts of At1g67300 and At1g79820 (6 TM).