Topic 6.5 ABC Transporters in Plants
The ATP-binding cassette (ABC) transporters are members of a large family of active transport proteins energized directly by ATP hydrolysis. Originally identified in microbial and animal cells, ABC transporters are able to use the energy of ATP hydrolysis directly to pump organic molecules (especially large anionic molecules) across a membrane. Like the P-type H+-ATP synthases, ABC transporters form a phosphorylated intermediate during catalysis, and they are therefore inhibited by vanadate.
A number of genes for ABC transporters have been cloned from various plants (Sanchez-Fernandez et al. 2001). Genes for ABC proteins are recognizable by a predicted protein structure consisting of one or two copies each of two structural units, a transmembrane domain (crossing the membrane several times) and a nucleotide-binding fold, which contains a characteristic signature sequence (Sanders and Bethke 2000). The transmembrane domain provides the pathway for solute transport and determines the molecular specificity of the transporter.
An ABC transporter has been found to secrete anti-fungal terpenes across the plasma membrane of tobacco cells. More commonly, ABC transporters are found at the tonoplast, where they are sometimes referred to as glutathione conjugate pumps, or GS-X pumps, since they often transport molecules that have been covalently attached to glutathione. Glutathione (GS) is a tripeptide (Glu-Cys-Gly) that functions as an important cellular antioxidant molecule (see textbook p. 369 for molecular structure). The vacuolar GS-X pumps of plant cells function in herbicide detoxification, protection against oxidative damage, pigment accumulation, and the storage of antimicrobial compounds.
A family of enzymes called glutathione transferases (GSTs) is responsible for attaching glutathione to the molecule to be transported. The common chemical determinant for GS conjugation is a carbon–carbon double bond adjacent to an electron-withdrawing group (CH2 = CH-Z) (Talalay et al. 1998). Compounds that contain this determinant include the anthocyanins (see textbook Figure 6.13), IAA (indoleacetic acid, or auxin), and various phenolic compounds. Metal-binding polypeptides called phytochelatins, which are synthesized from glutathione, are also transported into plant vacuoles along with their chelated heavy metals via GS-X pumps (Salt and Rauser 1995).