Dicarboxylate carrier protein

The chloroplast is encircled by a double membrane called the envelope. Of the two membranes, the inner is practically impermeable to hydrophylic compounds, such as Pj, phosphate esters, dicarboxylates, glucose and sucrose. Transport of certain of these metabolites is accomplished by carrier proteins, specific for groups of compounds. Individual carriers have been shown to facilitate the transport of Pj and phosphate esters, dicarboxylates, ATP and ADP, and glucose. >... 

Several additional mitochondrial carrier systems have been reconstituted into active form in proteoliposomes, using as starting material a crude neutral detergent mixture of membrane proteins from submitochondrial particles. These include the citrate transporter [203], the dicarboxylate carrier [204], and the carnitine transporters [202]. These reconstitution activities could be used as a basis for further purification and structural studies, but such studies have not yet been reported. 

For these reasons, it is accepted that a deficiency in renal absorption is responsible for the increased urinary excretion of amino acids in cystinuria. A defect in the reabsorption mechanism of four amino acids can be explained by the deletion of a single protein. Examination of the formula of the four amino acids spilled in cystinuria reveals a common denominator all four are diamino acids, therefore, it seems logical that their reabsorption involves a single carrier protein rich in dicarboxylic acids (aspartic and glutamic). 

Carrier-mediated membrane transport proteins on the RPE selectively transport nutrients, metabolites, and xenobiotics between the choriocapillaris and the cells of the distal retina, and include amino acid [33 35], peptide [36], dicarboxylate, glucose [37], monocarboxylic acid [38,39], nucleoside[40], and organic anion and organic cation [41] transporters. Membrane barriers such as the efflux pumps, including multidrug resistance protein (P-gp), and multidrug resistance-associated protein (MRP) pumps have also been identified on the RPE. Exploitation of these transport systems may be the key to circumventing the outer BRB. 

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