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Int. J. Mol. Sci. 2014, 15(11), 20413-20433; doi:10.3390/ijms151120413

An Uncleaved Signal Peptide Directs the Malus xiaojinensis Iron Transporter Protein Mx IRT1 into the ER for the PM Secretory Pathway

1
College of Life Science, Capital Normal University, Beijing 100048, China
2
Department of Biological Sciences, State University of New York, Buffalo, NY 14260, USA
3
School of Life Sciences, Tsinghua University, Beijing 100083, China
*
Author to whom correspondence should be addressed.
Received: 27 June 2014 / Revised: 11 October 2014 / Accepted: 20 October 2014 / Published: 7 November 2014
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Abstract

Malus xiaojinensis iron-regulated transporter 1 (Mx IRT1) is a highly effective inducible iron transporter in the iron efficient plant Malus xiaojinensis. As a multi-pass integral plasma membrane (PM) protein, Mx IRT1 is predicted to consist of eight transmembrane domains, with a putative N-terminal signal peptide (SP) of 1–29 amino acids. To explore the role of the putative SP, constructs expressing Mx IRT1 (with an intact SP) and Mx DsIRT1 (with a deleted SP) were prepared for expression in Arabidopsis and in yeast. Mx IRT1 could rescue the iron-deficiency phenotype of an Arabidopsis irt1 mutant, and complement the iron-limited growth defect of the yeast mutant DEY 1453 (fet3fet4). Furthermore, fluorescence analysis indicated that a chimeric Mx IRT1-eGFP (enhanced Green Fluorescent Protein) construct was translocated into the ER (Endoplasmic reticulum) for the PM sorting pathway. In contrast, the SP-deleted Mx DsIRT1 could not rescue either of the mutant phenotypes, nor direct transport of the GFP signal into the ER. Interestingly, immunoblot analysis indicated that the SP was not cleaved from the mature protein following transport into the ER. Taken together, data presented here provides strong evidence that an uncleaved SP determines ER-targeting of Mx IRT1 during the initial sorting stage, thereby enabling the subsequent transport and integration of this protein into the PM for its crucial role in iron uptake. View Full-Text
Keywords: iron transport; Malus xiaojinensis iron-regulated transporter 1 (Mx IRT1); Mx DsIRT1; multi-pass membrane protein; metal uptake; uncleaved signal peptide; ER (Endoplasmic reticulum) targeting iron transport; Malus xiaojinensis iron-regulated transporter 1 (Mx IRT1); Mx DsIRT1; multi-pass membrane protein; metal uptake; uncleaved signal peptide; ER (Endoplasmic reticulum) targeting
This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. (CC BY 4.0).

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MDPI and ACS Style

Zhang, P.; Tan, S.; Berry, J.O.; Li, P.; Ren, N.; Li, S.; Yang, G.; Wang, W.-B.; Qi, X.-T.; Yin, L.-P. An Uncleaved Signal Peptide Directs the Malus xiaojinensis Iron Transporter Protein Mx IRT1 into the ER for the PM Secretory Pathway. Int. J. Mol. Sci. 2014, 15, 20413-20433.

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