Dictyostelium Nramp1, structurally and functionally close to mammalian DMT1 transporter, mediates phagosomal iron efflux

Abstract

The Nramp (Slc11) protein family is widespread in bacteria and eukaryotes, and mediates transport of divalent metals across cellular membranes. The social amoeba Dictyostelium discoideum has two Nramp proteins. Nramp1, like its mammalian ortholog (SLC11A1), is recruited to phagosomal and macropinosomal membranes, and confers resistance to pathogenic bacteria. Nramp2 is located exclusively in the contractile vacuole membrane and controls, synergistically with Nramp1, iron homeostasis. It has long been debated whether mammalian Nramp1 mediates iron import or export from phagosomes. By selectively loading the iron-chelating fluorochrome calcein in macropinosomes, we show that Dictyostelium Nramp1 mediates iron efflux from macropinosomes in vivo. To gain insight in ion selectivity and the transport mechanism, the proteins were expressed in Xenopus oocytes. Using a novel assay with calcein, and electrophysiological and radiochemical assays, we show that Nramp1, similar to rat DMT1 (also known as SLC11A2), transports Fe2+ and manganese, not Fe3+ or copper. Metal ion transport is electrogenic and proton dependent. By contrast, Nramp2 transports only Fe2+ in a non-electrogenic and proton-independent way. These differences reflect evolutionary divergence of the prototypical Nramp2 protein sequence compared to the archetypical Nramp1 and DMT1 proteins.

A role for copper in protozoan grazing – two billion years selecting for bacterial copper resistance

“…The expression of NRAMP2 in Dictyostelium was significantly induced when ingesting E. coli as prey ( p < 0.05), while the expression of NRAMP1 was not affected, probably because its expression is time and dose dependent (Govoni et al ., ). Recently, it was shown that both NRAMP1 and NRAMP2 do not transport copper (Buracco et al ., ), which is consistent with the apparent insensitivity of both NRAMP1 and NRAMP2 to copper. Overall, genes involved in copper trafficking were more responsive to bacterial ingestion than NRAMP genes.…”

Metal homeostasis and resistance in bacteria

“…Similar to macrophages, some predatory protozoa use NRAMP-type transporters to deplete the phagosome of Fe(II) and Mn(II). For example, Nramp1 localizes to the phagosome 169 and Nramp2 to the contractile vacuole 170 in the social amoeba Dictoyostelium discoideum . Additionally, an ortholog of the Cu(I) transporter ATP7A in D. discoideum localized to both the cytoplasmic membrane and vacuolar structures, which indicates a possible use in flooding the phagocytic vacuole with Cu(I) 171 .…”

Mycobacterium marinum produces distinct mycobactin and carboxymycobactin siderophores to promote growth in broth and phagocytes

“…Pathogenic mycobacteria, such as M. marinum or M. tuberculosis , replicate in host cells in a distinct, membrane‐bound pathogen vacuole called MCV, followed by rupture of the vacuole membrane and exit of the bacteria to the cytoplasm (Cardenal‐Munoz et al, ; Koliwer‐Brandl et al, ; Russell, ). The MCV represents a metal ion‐poor environment (Chao et al, ), which is depleted of iron due to the activity of NRAMP‐1, a proton‐driven transporter of ferric iron (Bozzaro, Buracco, & Peracino, ; Buracco et al, ; Buracco, Peracino, Andreini, Bracco, & Bozzaro, ; Peracino, Buracco, & Bozzaro, ).…”

“…Russell, 2016). The MCV represents a metal ion-poor environment (Chao et al, 2019), which is depleted of iron due to the activity of NRAMP-1, a proton-driven transporter of ferric iron Buracco et al, 2015;Buracco, Peracino, Andreini, Bracco, & Bozzaro, 2017;.…”