Functional, structural and phylogenetic analysis of domains underlying the Al-sensitivity of the aluminium-activated malate/anion transporter, TaALMT1

Authors: LIGABA, AYALEW - Cornell University; DREYER, INGO - University Of Madrid; MARGARYAN, ARMINE - Yerevan State University; Schneider, David; Kochian, Leon; Pineros, Miguel

Submitted to: Plant Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/9/2013
Publication Date: 11/5/2013
Publication URL: http://onlinelibrary.wiley.com/doi/10.1111/tpj.12332/abstract
Citation: Ligaba, A., Dreyer, I., Margaryan, A., Schneider, D.J., Kochian, L.V., Pineros, M. 2013. Functional, structural and phylogenetic analysis of domains underlying the Al-sensitivity of the aluminium-activated malate/anion transporter, TaALMT1. Plant Journal. 76(6):766-780. DOI: 10.1111/tpj.12332

Interpretive Summary: Over 20% of the US land area and approximately 50% of the world’s arable lands are comprised of acidic soils (pH < 5). On these acid soils, aluminum (Al) toxicity is the primary factor limiting agricultural productivity, as toxic Al damages plant root systems, ultimately resulting in a reduction of crop yields. Since a significant fraction of the worldwide acid soils are found in the tropics/subtropics regions of many developing countries, Al toxicity limits agricultural productivity in the very areas where food security is most tenuous. Given the worldwide agriculture limitation due to Al toxicity, there is a sizable interest and research effort by universities, government agencies, and international agriculture organizations placed on the identification of genes underlying tolerance to Al toxicity, with the ultimate goal to improve crop Al tolerance via molecular breeding and biotechnology. The release of Al detoxifying organic acids from the root apex in response to Al-stress constitutes a widespread Al-tolerance mechanism by which plant roots are able to ameliorate the toxic levels of Al surrounding the growing root. Several genes encoding membrane proteins involved in the transport and the release of these organic acids from root cells have been identified and cloned. However, the knowledge regarding the structure and functional characteristics of these membrane transport proteins is yet very limited. In this study, we used molecular and biophysical approaches to characterize the structural and functional properties of TaALMT1, the membrane transport protein mediating the organic acid release that underlies Al-tolerance in wheat. We have identified key structural regions of this protein defining some of the functional properties of this protein that allow it to confer Al tolerance in crop plants, which in turn underlie their physiological role in the whole plant response to Al toxicity. These findings are highly important as they increase our understanding of how protein mediating the Al-tolerance response in crops actually functions, which will provide the information to engineer proteins that confer greater Al tolerance. Thus this research will allow us to build a platform for improving cereal crop production on acid soils via molecular breeding approaches.

Technical Abstract: TaALMT1 (Triticum aestivum Aluminum Activated Malate Transporter) is the founding member of a novel gene family of anion transporters (ALMTs) that mediate the efflux of organic acids. A small subgroup of root-localized ALMTs, including TaALMT1, is physiologically associated with in planta aluminum (Al)-resistance. TaALMT1 exhibits significant enhancement of transport activity in response to extracellular Al. Here we have integrated structure-function analyses of structurally altered TaALMT1 proteins expressed in Xenopus oocytes, with phylogenic analyses of the ALMT family. Our aim to reexamine the role of protein domains in terms of their potential involvement in the Al-dependent enhancement (i.e. Al-responsiveness) of TaALMT1 transport activity, as well as the roles of all 43 negatively charged. Our results indicate that the N-domain, predicted to form the conductive pathway, can mediate ion transport even in the absence of the C-domain. Segments in both domains, however, are involved in Al3+-sensing. We identified two regions, one at the N-terminus and a hydrophobic region at the C-terminus, which jointly contribute to the Al-response phenotype. Interestingly, the characteristic motif at the N-terminus appears to be specific for Al-responsive ALMTs. Our study highlights the need for including a comprehensive phylogenetic analysis for inferences drawn from function-structure analyses, as a significant proportion of the functional changes observed for TaALMT1 are most likely the result of alterations in the overall structural integrity of ALMT family proteins rather than modifications of specific sites involved in Al3+-sensing.