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

Spider Silk-CBD-Cellulose Nanocrystal Composites: Mechanism of Assembly

The fabrication of cellulose-spider silk bio-nanocomposites comprised of cellulose nanocrystals (CNCs) and recombinant spider silk protein fused to a cellulose binding domain (CBD) is described. Silk-CBD successfully binds cellulose, and unlike recombinant silk alone, silk-CBD self-assembles into microfibrils even in the absence of CNCs. Silk-CBD-CNC composite sponges and films show changes in internal structure and CNC alignment related to the addition of silk-CBD. The silk-CBD sponges exhibit improved thermal and structural characteristics in comparison to control recombinant spider silk sponges. The glass transition temperature (Tg) of the silk-CBD sponge was higher than the control silk sponge and similar to native dragline spider silk fibers. Gel filtration analysis, dynamic light scattering (DLS), small angle X-ray scattering (SAXS) and cryo-transmission electron microscopy (TEM) indicated that silk-CBD, but not the recombinant silk control, formed a nematic liquid crystalline phase similar to that observed in native spider silk during the silk spinning process. Silk-CBD microfibrils spontaneously formed in solution upon ultrasonication. We suggest a model for silk-CBD assembly that implicates CBD in the central role of driving the dimerization of spider silk monomers, a process essential to the molecular assembly of spider-silk nanofibers and silk-CNC composites

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

Crystallographic data for ArsTM.

<p>Crystallographic data for ArsTM.</p

Circular dichroism measurements of ArsTM (purple) and MamAΔ41 proteins from RS-1 (Blue) Mbav (orange), AMB-1 (green) and MSR-1 (red).

<p>(A) Circular dichroism spectra. (B) Circular dichroism melting curve measurements at 222 nm. Wild type MamAΔ41_RS-1 presents the lowest thermostability, with a melting temperature of ~40°C, while the triple mutated MamAΔ41_RS-1 (ArsTM) exhibits a slightly increased thermostability with a melting temperature of ~ 51°C. MamAΔ41_AMB-1, MamAΔ41_MSR-1 and MamAΔ41_Mbav present melting temperatures of ~51, 53 and 65°C, respectively.</p