Arabidopsis HMA2, a Divalent Heavy Metal-Transporting PIB-Type ATPase, Is Involved in Cytoplasmic Zn2+ Homeostasis

Abstract

Zn 21 plays a critical role in plants as an essential component of key enzymes (Cu-Zn superoxide dismutase, alcohol dehydrogenase, RNA polymerase, etc.) and DNA-binding proteins (Marschner, 1995;Guerinot and Eide, 1999). Zn 21 deficiency leads to a reduction of internodal growth with a consequent rosette-like development and also produces an impaired response to oxidative stress, likely due to a reduction in superoxide dismutase levels (Hacisalihoglu et al., 2003). Thus, Zn 21 deficiency is a significant agricultural problem, particularly in cereals, limiting crop production and quality (Guerinot and Eide, 1999;Hacisalihoglu et al., 2003 Silva and Williams, 2001;Hall, 2002). Consequently, plants and other organisms have developed molecular chaperones, chelators, and specific transmembrane transporters to (1) absorb and distribute metal micronutrients throughout the entire organism and (2) prevent high cytoplasmic concentrations of free heavy metals ions (Fox and Guerinot, 1998;Rauser, 1999;Guerinot, 2000;Williams et al., 2000;Clemens, 2001;Cobbett and Goldsbrough, 2002;Hall, 2002). These processes require the metal to be transported through permeability barriers and compartments delimited by lipid membranes. Several types of heavy metal transmembrane transporters have been identified in plants (Rea, 1999;Guerinot, 2000;Maser et al., 2001;Baxter et al., 2003). Since metal ions must be transported against electrochemical gradients at some point during plant distribution, metal pumps involved in contragradient transport should play key roles in metal homeostasis. The presence of plant genes encoding proteins that specifically perform this function (mainly P IB -ATPases) is known and their potential importance has been repeatedly noted (Williams et al., 2000;Clemens, 2001;Hall, 2002 (Lutsenko and Kaplan, 1995;Axelsen and Palmgren, 1998;Argü ello, 2003). Initial reports named these proteins CPxATPases (Solioz and Vulpe, 1996). They confer metal tolerance to microorganisms (Solioz and Vulpe, 1996;Rensing et al., 1999) and are essential for the absorption, distribution, and bioaccumulation of metal micronutrients by higher organisms (Bull and Cox, 1994; 1 This work was supported by the U.S. Department of Agriculture (grant no. 2001-35106-10736) and by the National Science Foundation (grant no. MCM-0235165).* Corresponding author; e-mail arguello@wpi.edu; fax 508-831-5933.Article, publication date, and citation information can be found at www.plantphysiol.org/cgi

A Novel Regulatory Metal Binding Domain Is Present in the C Terminus of Arabidopsis Zn2+-ATPase HMA2

“…This Zn 2ϩ -ATPase drives the efflux of metals out of the cell and is activated by Zn 2ϩ and Cd 2ϩ with quite low apparent affinities (0.1-0.2 M). Analysis of A. thaliana hma2 knock-out mutants revealed a significant increase in whole plant Zn 2ϩ and Cd 2ϩ levels (10). This observation along with the plasma membrane localization and strong expression in the plant vasculature suggests that HMA2 is responsible for Zn 2ϩ uploading into the phloem (10,11).…”

“…Analysis of A. thaliana hma2 knock-out mutants revealed a significant increase in whole plant Zn 2ϩ and Cd 2ϩ levels (10). This observation along with the plasma membrane localization and strong expression in the plant vasculature suggests that HMA2 is responsible for Zn 2ϩ uploading into the phloem (10,11). P 1B -type ATPases have 6 -8 transmembrane fragments responsible for metal translocation and a large cytoplasmic loop involved in ATP binding and hydrolysis (1)(2)(3).…”

“…Distinctly, only two Cu ϩ -ATPase isoforms are found in other eukaryotes (1)(2)(3). We recently characterized the functional role of Arabidopsis thaliana HMA2 (10). This Zn 2ϩ -ATPase drives the efflux of metals out of the cell and is activated by Zn 2ϩ and Cd 2ϩ with quite low apparent affinities (0.1-0.2 M).…”

Strategies to increase zinc deficiency tolerance and homeostasis in plants

“…The AtHMA2 gene encodes a Zn +2 -ATPase located in the plasma membrane. Expression is induced by cadmium and zinc (Eren and Argüello, 2004). The AtHMA3 protein possibly mediates zinc hyperaccumulation, since zinc hyperaccumulator species show higher expression of HMA3 than the non-hyperaccumulator ones, such as Arabidopsis (Becher et al, 2004;van de Mortel et al, 2006;Hassan and Aarts, 2011).…”

“…Arabidopsis has eight genes encoding P 1B -ATPases that differ in their structure, function, and regulation (Eren and Argüello, 2004). Among these, HMA1, HMA2, HMA3, and HMA4 are involved in zinc transport (Hussain et al, 2004).…”