ABC Transporters & Auxin

 

ATP Binding Cassette (ABC) transporters comprise a superfamily of membrane proteins found in all manner of organisms. Probably the most studied member is mammalian ABCB1 because it is highly expressed in cancer cells that have become resistant to chemotherapeutic agents (1). The protein is believed to pump out lipophilic drugs, thus rendering highly expressing cells multidrug resistant. The Arabidopsis genome encodes several ABC transporters belonging to the same subfamily as human ABCB1 (2). Bosl Noh, then a graduate student, isolated one of them in a screen for genes differentially expressed in plants treated with an anion-channel inhibitor that we were using to study blue light signal transduction (3,4). Mutants lacking the gene which we called MDR1 (now ABCB19) displayed cotyledon epinasty reminiscent of auxin-treated seedlings. Angus Murphy isolated the same gene in a search for NPA-binding proteins. Working together, we found that polar auxin transport was severely impaired in these mutants (5). Now, eleven years later, there is a large amount of evidence showing an important role for B-group ABC transporters in the polar transport of auxin (6). The mechanism is not entirely clear but the prevailing view is that ABCB1 and ABCB19 pump auxin out of cells. Yet, auxin transport out of cells should not require an ATP-driven pump because the auxin anion which predominates at the neutral pH of the cytoplasm is thermodynamically poised to leave the cell through passive channels (7). Also, ABCB proteins are symmetrically distributed in cells so what gives directional bias to ABCB-mediated efflux is not obvious. Yet, polar auxin transport through tissues is unequivocally impaired in mutants such as abcb19. Perhaps ABCB proteins enhance auxin efflux mediated by polarly-localized passive transporters such as PIN1. We are currently investigating ABCB and PIN transporters with the biophysically rigorous patch clamp technique. Our first evidence of ion channel activity for ABCB19 and its blockage by the inhibitor used in the initial molecular screen has been published (8).

We genetically manipulate ABCB transporters in Arabidopsis or an immunophilin gene (TWD1) on which trafficking of the ABCB transporters depends (9) in order to affect auxin flow and distribution in seedlings. Then we use custom machine-vision tools (see the Phytomorph project) to quantify how altered auxin flows affect seedling development. For example, loss of ABCB19 causes sporadic redirection of the primary root tip as it grows, apparently due to imbalances in auxin flows near the root apex (10). In more mature regions of the root, loss of ABCB19 slows elongation of newly-emerged lateral roots by reducing auxin levels at the lateral root apex (11). The related ABCB4 protein controls flow of auxin from the primary root tip toward the base, which affects gravitropic curvature distribution (10). At the shoot apex, flow of auxin into expanding cotyledons depends on ABCB19 and is required for full blade expansion (12). Overexpressing ABCB19 increases auxin in the hypocotyl, delays opening of the apical hook in response to light, and affects growth control exerted by cryptochrome and phytochrome photoreceptors (13). The long term goal of our ABCB research is to generate quantitative models that relate patterns of auxin flow to seedling growth and morphogenesis.

 

The National Science Foundation funds this work.

 

Edgar Spalding

Spalding Lab

Mouse ABCB1a structure solved


See the article by Aller et al. (2009) Structure of P-glycoprotein reveals a molecular basis for poly-specific drug binding. Science 323: 1718-1722.