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Title: MOLECULAR ANALYSIS OF PLANT SUGAR AND AMINO ACID TRANSPORTERS

Author
item BUSH, DANIEL
item CHIOU, TZYY-JEN - UNIV OF ILLINOIS URBANA
item CHEN, LISHAN - UNIV OF ILLINOIS URBANA

Submitted to: Journal of Experimental Botany
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/19/1995
Publication Date: N/A
Citation: N/A

Interpretive Summary: Plant leaves capture light energy from the sun and transform that energy into a useful form in the process called photosynthesis. The primary product of photosynthesis in most agricultural plants is sugar. Generally, eighty percent of the sugar formed in the leaf is transported to plant tissues of agricultural significance (fruit, grain, and tubers) that do not capture light energy. Amino acids are also assimilated in the leaf and subsequently transported to harvested tissues. Alterations in the transport processes associated with resource redistribution are known to significantly affect crop productivity and nutritional quality. The work reported here describes the discovery of a sugar transport protein that is targeted to the vacuole of sugar beet cells. We also report on amino acid transport mutants that reveal new insight into the molecular function of a critical amino acid transport protein. Knowledge of how these transport systems work will be used to develop novel strategies for increasing crop productivity and/ or nutritive value.

Technical Abstract: In the past seven years significant progress has been made in describing the transport properties and molecular genetics of sugar and amino acid transport systems that mediate assimilate partitioning in the plant. 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. For example, at least twelve genes encoding putative sugar transporters in the Major Facilitator Super family have been identified. In the results reported here, we demonstrated that a sugar beet member of this superfamily is targeted to the tonoplast membrane, thus implicating that porter in intracellular sugar partitioning. To further our understanding of amino acid transport, we've initiated a detailed analysis of NAT2 as a prototypical example of this class of amino acid carriers. We are using mutagenesis to identify important amino acid residues and protein domains. We report here that site-directed mutants of His(336) show that this is an essential residue required for transport function. In random mutants, we have identified substrate binding mutants and porters that have altered proton coupling. Taken together, these mutants we allow us to build the first molecular model of a plant amino acid transporter.