MamA as a Model Protein for Structure-Based Insight into the Evolutionary Origins of Magnetotactic Bacteria

International audienceMamA is a highly conserved protein found in magnetotactic bacteria (MTB), a diverse group of prokaryotes capable of navigating according to magnetic fields - an ability known as magnetotaxis. Questions surround the acquisition of this magnetic navigation ability; namely, whether it arose through horizontal or vertical gene transfer. Though its exact function is unknown, MamA surrounds the magnetosome, the magnetic organelle embedding a biomineralised nanoparticle and responsible for magnetotaxis. Several structures for MamA from a variety of species have been determined and show a high degree of structural similarity. By determining the structure of MamA from Desulfovibrio magneticus RS-1 using X-ray crystallography, we have opened up the structure-sequence landscape. As such, this allows us to perform structural-and phylogenetic-based analyses using a variety of previously determined MamA from a diverse range of MTB species across various phylogenetic groups. We found that MamA has remained remarkably constant throughout evolution with minimal change between different taxa despite sequence variations. These findings, coupled with the generation of phylogenetic trees using both amino acid sequences and 16S rRNA, indicate that magnetotaxis likely did not spread via horizontal gene transfer and instead has a significantly earlier, primordial origin

Genetic Manipulation of Iron Biomineralization Enhances MR Relaxivity in a Ferritin-M6A Chimeric Complex

Ferritin has gained significant attention as a potential reporter gene for in vivo imaging by magnetic resonance imaging (MRI). However, due to the ferritin ferrihydrite core, the relaxivity and sensitivity for detection of native ferritin is relatively low. We report here on a novel chimeric magneto-ferritin reporter gene – ferritin-M6A – in which the magnetite binding peptide from the magnetotactic bacteria magnetosome-associated Mms6 protein was fused to the C-terminal of murine h-ferritin. Biophysical experiments showed that purified ferritin-M6A assembled into a stable protein cage with the M6A protruding into the cage core, enabling magnetite biomineralisation. Ferritin-M6A-expressing C6-glioma cells showed enhanced (per iron) r2 relaxivity. MRI in vivo studies of ferritin-M6A-expressing tumour xenografts showed enhanced R2 relaxation rate in the central hypoxic region of the tumours. Such enhanced relaxivity would increase the sensitivity of ferritin as a reporter gene for non-invasive in vivo MRI-monitoring of cell delivery and differentiation in cellular or gene-based therapies

Cation Diffusion Facilitators Transport Initiation and Regulation Is Mediated by Cation Induced Conformational Changes of the Cytoplasmic Domain

Cation diffusion facilitators (CDF) are part of a highly conserved protein family that maintains cellular divalent cation homeostasis in all domains of life. CDF's were shown to be involved in several human diseases, such as Type-II diabetes and neurodegenerative diseases. In this work, we employed a multi-disciplinary approach to study the activation mechanism of the CDF protein family. For this we used MamM, one of the main ion transporters of magnetosomes - bacterial organelles that enable magnetotactic bacteria to orientate along geomagnetic fields. Our results reveal that the cytosolic domain of MamM forms a stable dimer that undergoes distinct conformational changes upon divalent cation binding. MamM conformational change is associated with three metal binding sites that were identified and characterized. Altogether, our results provide a novel auto-regulation mode of action model in which the cytosolic domain's conformational changes upon ligand binding allows the priming of the CDF into its transport mode

The dual role of MamB in magnetosome membrane assembly and magnetite biomineralization

Magnetospirillum gryphiswaldense MSR‐1 synthesizes membrane‐enclosed magnetite (Fe$_3$O$_4$) nanoparticles, magnetosomes, for magnetotaxis. Formation of these organelles involves a complex process comprising key steps which are governed by specific magnetosome‐associated proteins. MamB, a cation diffusion facilitator (CDF) family member has been implicated in magnetosome‐directed iron transport. However, deletion mutagenesis studies revealed that MamB is essential for the formation of magnetosome membrane vesicles, but its precise role remains elusive. In this study, we employed a multi‐disciplinary approach to define the role of MamB during magnetosome formation. Using site‐directed mutagenesis complemented by structural analyses, fluorescence microscopy and cryo‐electron tomography, we show that MamB is most likely an active magnetosome‐directed transporter serving two distinct, yet essential functions. First, MamB initiates magnetosome vesicle formation in a transport‐independent process, probably by serving as a landmark protein. Second, MamB transport activity is required for magnetite nucleation. Furthermore, by determining the crystal structure of the MamB cytosolic C‐terminal domain, we also provide mechanistic insight into transport regulation. Additionally, we present evidence that magnetosome vesicle growth and chain formation are independent of magnetite nucleation and magnetic interactions respectively. Together, our data provide novel insight into the role of the key bifunctional magnetosome protein MamB, and the early steps of magnetosome formation

Distinct features of the Leishmania cap-binding protein LeishIF4E2 revealed by CRISPR-Cas9 mediated hemizygous deletion.

Leishmania parasites cycle between sand-fly vectors and mammalian hosts adapting to alternating environments by stage-differentiation accompanied by changes in the proteome profiles. Translation regulation plays a central role in driving the differential program of gene expression since control of gene regulation in Leishmania is mostly post-transcriptional. The Leishmania genome encodes six eIF4E paralogs, some of which bind a dedicated eIF4G candidate, and each eIF4E is assumed to have specific functions with perhaps some overlaps. However, LeishIF4E2 does not bind any known eIF4G ortholog and was previously shown to comigrate with the polysomal fractions of sucrose gradients in contrast to the other initiation factors that usually comigrate with pre-initiation and initiation complexes. Here we deleted one of the two LeishIF4E2 gene copies using the CRISPR-Cas9 methodology. The deletion caused severe alterations in the morphology of the mutant cells that became round, small, and equipped with a very short flagellum that did not protrude from its pocket. Reduced expression of LeishIF4E2 had no global effect on translation and growth, unlike other LeishIF4Es; however, there was a change in the proteome profile of the LeishIF4E2(+/-) cells. Upregulated proteins were related mainly to general metabolic processes including enzymes involved in fatty acid metabolism, DNA repair and replication, signaling, and cellular motor activity. The downregulated proteins included flagellar rod and cytoskeletal proteins, as well as surface antigens involved in virulence. Moreover, the LeishIF4E2(+/-) cells were impaired in their ability to infect cultured macrophages. Overall, LeishIF4E2 does not behave like a general translation factor and its function remains elusive. Our results also suggest that the individual LeishIF4Es perform unique functions

Disease-Homologous Mutation in the Cation Diffusion Facilitator Protein MamM Causes Single-Domain Structural Loss and Signifies Its Importance

Cation diffusion facilitators (CDF) are highly conserved, metal ion efflux transporters that maintain divalent transition metal cation homeostasis. Most CDF proteins contain two domains, the cation transporting transmembrane domain and the regulatory cytoplasmic C-terminal domain (CTD). MamM is a magnetosome-associated CDF protein essential for the biomineralization of magnetic iron-oxide particles in magnetotactic bacteria. To investigate the structure-function relationship of CDF cytoplasmic domains, we characterized a MamM M250P mutation that is synonymous with the disease-related mutation L349P of the human CDF protein ZnT-10. Our results show that the M250P exchange in MamM causes severe structural changes in its CTD resulting in abnormal reduced function. Our in vivo, in vitro and in silico studies indicate that the CTD fold is critical for CDF proteins’ proper function and support the previously suggested role of the CDF cytoplasmic domain as a CDF regulatory element. Based on our results, we also suggest a mechanism for the effects of the ZnT-10 L349P mutation in human

Correction: MamA as a Model Protein for Structure-Based Insight into the Evolutionary Origins of Magnetotactic Bacteria

<p>Correction: MamA as a Model Protein for Structure-Based Insight into the Evolutionary Origins of Magnetotactic Bacteria</p

ArsTM crystal packing, asymmetric unit composition and overall structure.

<p><b><i>(A)</i></b> ArsTM crystal packing and asymmetric unit composition. The molecules are shown in three rotation-related views. <b><i>(B)</i></b> Overlay of all six ArsTM monomers reveals the high degree of structural similarity. The representative ArsTM monomer contains five sequential TPR motifs. The molecule is shown in two views, related by a 90° rotation. <b><i>(C)</i></b> An overlay of representative monomers from ArsTM (green), MamAΔ41_Mbav (PDB ID: 3VTX, orange) and <i>Magnetospirillum</i> species MamAΔ41_AMB-1 (PDB ID: 3AS5 chain A and B in light pink and brown, respectively) related by a 180° rotation. A high structural similarity of MamAΔ41 between the species can be observed, apart from the helical conformation of the identical His-tag linker sequence remaining after thrombin proteolysis (H11: ELALVPR) seen in the 3AS5 chain B and 3VTX. In addition, a light flexibility is observed at the NTD of the monomers. All images were produced by PyMOL.</p

ConSurf analysis shows a high degree of structural conservation between ArsTM and MamA from alternative species, though this is largely limited to the concave surface and a cluster around the fifth TPR and the C-terminal.

<p>All sequences used in this analysis are shown in the multiple sequence alignment of <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0130394#pone.0130394.s002" target="_blank">S2 Fig</a>.</p

<i>(A)</i> Three ArsTM monomers form the trimeric ring.

<p>This flat ring encloses a ~15 Å diameter inner void. <b><i>(B)</i></b> Interaction surface between two monomers that form the trimeric ring. The forces that stabilise the trimeric ring include salt bridges as well as hydrophobic interactions between the N-terminal of a single monomer to the C-terminal of a nearby monomer in a continuous manner.</p