Golgi Localized Barley MTP8 Proteins Facilitate Mn Transport

Abstract

Many metabolic processes in plants are regulated by manganese (Mn) but limited information is available on the molecular mechanisms controlling cellular Mn homeostasis. In this study, a yeast assay was used to isolate and characterize two genes, MTP8.1 and MTP8.2, which encode membrane-bound proteins belonging to the cation diffusion facilitator (CDF) family in the cereal species barley (Hordeum vulgare). Transient expression in onion epidermal cells showed that MTP8.1 and MTP8.2 proteins fused to the green fluorescent protein (GFP) are localized to Golgi. When heterologously expressed in yeast, MTP8.1 and MTP8.2 were found to be Mn transporters catalysing Mn efflux in a similar manner as the Golgi localized endogenous yeast protein Pmr1p. The level of MTP8.1 transcripts in barley roots increased with external Mn supply ranging from deficiency to toxicity, while MTP8.2 transcripts decreased under the same conditions, indicating non-overlapping functions for the two genes. In barley leaves, the expression of both MTP8 genes declined in response to toxic Mn additions to the roots suggesting a role in ensuring proper delivery of Mn to Golgi. Based on the above we suggest that barley MTP8 proteins are involved in Mn loading to the Golgi apparatus and play a role in Mn homeostasis by delivering Mn to Mn-dependent enzymes and/or by facilitating Mn efflux via secretory vesicles. This study highlights the importance of MTP transporters in Mn homeostasis and is the first report of Golgi localized Mn2+ transport proteins in a monocot plant species.

The Vacuolar Manganese Transporter MTP8 Determines Tolerance to Iron Deficiency-Induced Chlorosis in Arabidopsis

“…Mn specificity and vacuolar localization were recently also demonstrated for MTP8 orthologs in cucumber (Cucumis sativus; Migocka et al, 2014) and rice (Oryza sativa; Chen et al, 2013). The function of MTP8 to sequester Mn into vacuoles, therefore, is widely conserved across species, with the possible exception of barley (Hordeum vulgare), where MTP8 homologs have been localized to the Golgi apparatus (Pedas et al, 2014). However, the circumstances under which this function is employed may differ.…”

Direct Comparison of Manganese Detoxification/Efflux Proteins and Molecular Characterization of ZnT10 Protein as a Manganese Transporter

“…The plant MTP8, which transports manganese but not zinc, has an Asp amino acid at this position. MTP8 has four Asp residues, including the Asp residue in TMDs II and V, which are thought to form an intramembranous tetrahedral metal coordination site (59,60). Thus, the finding that the Asn residue at this position of TMD II regulates manganese transport ability is unique, and thus clarification of its specific importance should provide insight into understanding the metal coordination properties in TMDs of ZnT transporters and their homologs.…”

Intracellular Distribution of Manganese by the Trans-Golgi Network Transporter NRAMP2 Is Critical for Photosynthesis and Cellular Redox Homeostasis

“…Inactivation of ECA1 and ECA3 alters the tolerance of plants to suboptimal Mn conditions (Wu et al, 2002;Mills et al, 2008). Moreover, MTP11, another Mn-CDF member from Arabidopsis, and its barley (Hordeum vulgare) homolog Hv-MTP8, are localized in Golgi /PVC compartments where they control plant tolerance to high Mn through a mechanism that may involve vesicular trafficking to the vacuole and/or secretion to the extracellular space (Delhaize et al, 2007;Peiter et al, 2007;Pedas et al, 2014). Finally, Smf2p, an NRAMP family transporter, is also present in the secretory pathway in yeast, although its target compartment (reported as being intracellular vesicles) still awaits formal identification (Luk and Culotta, 2001).…”