Molecular analysis of plant sugar and amino acid transporters

Although plants are photoautotrophic organisms, they are composed of many heterotrophic tissue systems that must import sugars and amino acids in a process known as assimilate partitioning. While the general features of assimilate partitioning were well described by the early 1980s, little was known about the various transport proteins involved in this essential activity associated with multicellular growth. In the past seven years, however, significant progress has been made in describing the transport properties and molecular genetics of these critical transport systems. Initially, these porters were well characterized using purified membrane vesicles and imposed proton electrochemical potential differences. This approach allowed for a detailed analysis of their transport kinetics, bioenergetics, and substrate specificity. Subsequently, several transporters were cloned using differential hybridization and functional complementation of yeast transport mutants. At first, isolation of transporter genes seemed to simplify our understanding by filling in gaps associated with transporter function. However, it has become increasingly clear that assimilate partitioning has many levels of complexity yet to be penetrated. This is best illustrated by the large number of carriers cloned. For example, at least 12 genes encoding putative sugar transporters in the Major Facilitator Superfamily have been identified in plants. Moreover, recent work in this laboratory has demonstrated that sugar beet member of this superfamily is targeted to the tonoplast membrane, thus implicating that porter in intracellular sugar partitioning. Similar complexity is emerging in the number of plant amino acid transporters identified. Therefore, a detailed analysis of AAP1/NAT2 as a prototypical example of one class of amino acid carriers has been initiated. The topology of this porter is being mapped, and site-directed and random mutagenesis are being used to identify functionally important amino acid residues and protein domains. The major challenges facing this field are to determine the unique contributions made by these many transport systems, to understand their structure and function relationships and, ultimately, to identify the mechanisms which regulate activity and integrate assimilate distribution in the plant as a multicellular organism.