Inner Envelope CHLOROPLAST MANGANESE TRANSPORTER 1 Supports Manganese Homeostasis and Phototrophic Growth in Arabidopsis

Abstract

Manganese (Mn) is an essential catalytic metal in the Mn-cluster that oxidizes water to produce oxygen during photosynthesis. However, the transport protein(s) responsible for Mn import into the chloroplast remains unknown. Here, we report the characterization of Arabidopsis CMT1 (Chloroplast Manganese Transporter 1), an evolutionarily conserved protein in the Uncharacterized Protein Family 0016 (UPF0016), that is required for manganese accumulation into the chloroplast. CMT1 is expressed primarily in green tissues, and its encoded product is localized in the inner envelope membrane of the chloroplast. Disruption of CMT1 in the T-DNA insertional mutant cmt1-1 resulted in stunted plant growth, defective thylakoid stacking, and severe reduction of photosystem II complexes and photosynthetic activity. Consistent with reduced oxygen evolution capacity, the mutant chloroplasts contained less manganese than the wild-type ones. In support of its function as a Mn transporter, CMT1 protein supported the growth and enabled Mn accumulation in the yeast cells of Mn-uptake deficient mutant (Δsmf1). Taken together, our results indicate that CMT1 functions as an inner envelope Mn transporter responsible for chloroplast Mn uptake.

Homologous Proteins of the Manganese Transporter PAM71 Are Localized in the Golgi Apparatus and Endoplasmic Reticulum

“…The genome of Arabidopsis contains three sequences that code for proteins with high similarity to PAM71 and CMT1, two previously characterized Mn transporters localized in distinct chloroplast membranes [8][9][10]. Like PAM71 and CMT1, the three predicted protein sequences contain the two highly conserved E-x-G-D-(KR)-(TS) motifs, with two negatively charged acidic residues in TM1 and TM4, which provide a suitable environment for the passage of Mn 2+ ions and maybe other cations through membranes.…”

“…The outer envelope membrane is non-selectively permeable to most ions, while the inner envelope and the thylakoid membrane possess specific transport proteins. At the inner envelope membrane, chloroplast manganese transporter 1 (CMT1) represents the first candidate for the long-sought Mn import protein [8,9]. CMT1 transports Mn from the cytosol into the chloroplast stroma.…”

Manganese in Plants: From Acquisition to Subcellular Allocation

“…Disruption of AtBICAT2 resulted in strong chlorosis, severely impaired plant growth, defective thylakoid stacking, and severe reduction of PS II complexes, resulting in diminished photosynthetic activity (Eisenhut et al, 2018;Frank et al, 2019). Consistent with a reduced oxygen evolution capacity, bicat2 mutant chloroplasts contained less Mn than those of the wild type (Zhang et al, 2018). Also, mutants were defective in Ca 2+ uptake across the chloroplast envelope and showed a strongly dampened dark-induced Ca 2+ signal in the stroma (Frank et al, 2019).…”

Chloroplast Calcium Signaling in the Spotlight

“…Altogether, these channels/ transporters can be involved in the formation of the plastidial Ca 2+ transients, along with the putative calcium-transporting protein PAM71/BICAT (Frank et al, 2019). However, this latter protein seems to play a prevalent role in manganese homeostasis rather than in calcium homeostasis (Schneider et al, 2016; Dammann et al, 2003;Nühse et al, 2003;Nühse et al, 2004;Benschop et al, 2007;Chen et al, 2010;Keinath et al, 2010;Zhang and Peck, 2011;Elmore et al, 2012;Zargar et al, 2015;Zou et al, 2015 At5g24430 AtCRK4 Calcium Dunkley et al, 2006;Benschop et al, 2007;Anil et al, 2008;Zhang and Peck, 2011;Nikolovski et al, 2012;Heard et al, 2015 At5g53010 Calciumtransporting ATPase, putative Ferro et al, 2010;Zybailov et al, 2008;Ferro et al, 2010;Tomizioli et al, 2014;Eisenhut et al, 2018;Zhang et al, 2018;Frank et Zhang et al, 2018). In addition to Ca 2+ channels and transporters, Ca 2+ sensors, namely 21 proteins, are predicted to be located in plastids.…”