Self-recognition mechanism of MamA, a magnetosome-associated TPR-containing protein, promotes complex assembly

Abstract

The magnetosome, a biomineralizing organelle within magnetotactic bacteria, allows their navigation along geomagnetic fields. Magnetosomes are membrane-bound compartments containing magnetic nanoparticles and organized into a chain within the cell, the assembly and biomineralization of magnetosomes are controlled by magnetosome-associated proteins. Here, we describe the crystal structures of the magnetosome-associated protein, MamA, from Magnetospirillum magneticum AMB-1 and Magnetospirillum gryphiswaldense MSR-1. MamA folds as a sequential tetra-trico-peptide repeat (TPR) protein with a unique hook-like shape. Analysis of the MamA structures indicates two distinct domains that can undergo conformational changes. Furthermore, structural analysis of seven crystal forms verified that the core of MamA is not affected by crystallization conditions and identified three protein–protein interaction sites, namely a concave site, a convex site, and a putative TPR repeat. Additionally, relying on transmission electron microscopy and size exclusion chromatography, we show that highly stable complexes form upon MamA homooligomerization. Disruption of the MamA putative TPR motif or N-terminal domain led to protein mislocalization in vivo and prevented MamA oligomerization in vitro. We, therefore, propose that MamA self-assembles through its putative TPR motif and its concave site to create a large homooligomeric scaffold which can interact with other magnetosome-associated proteins via the MamA convex site. We discuss the structural basis for TPR homooligomerization that allows the proper function of a prokaryotic organelle.

Genetic Dissection of the mamAB and mms6 Operons Reveals a Gene Set Essential for Magnetosome Biogenesis in Magnetospirillum gryphiswaldense

“…The TPR domain-containing protein MamA was speculated to play a role in activation of biomineralization (5). It was suggested that MamA self-assembles through its putative TPR domain and concave site to form a large homooligomeric scaffold surrounding the magnetosomes (46,47), whereas its convex site interacts with other magnetosome-associated proteins, like MamC, and several unidentified proteins (46,47). However, as in M. magneticum, the deletion of mamA in M. gryphiswaldense had only a weak effect (5), suggesting that these interactions are not essential or can be partly compensated by other proteins.…”

“…MamQ (23). MamA forms a multiprotein complex surrounding the magnetosome membrane (47), and MamE is involved in localization of magnetosome proteins by a protease-independent process (32). The heterodimer of the CDF transporters MamB and MamM transports ferrous iron into the magnetosome vesicles (35), and ferric iron is taken up by MamH and MamZ (20) or formed by oxidation of ferrous iron within the vesicles.…”

Ecology, Diversity, and Evolution of Magnetotactic Bacteria

“…Experimental evidence, however, suggests that MamI and MamL might be involved in the invagination of the magnetosome membrane, the first step in magnetosome formation (174), although the specific mechanism by which this is mediated remains unclear. The functional domains that have been identified in the remainder of this core set of proteins through in silico and/or experimental evidence include (i) one tetratricopeptide repeat (TPR) (e.g., MamA) that participates in assembly of the magnetosome membrane through protein-protein interactions (203,204), (ii) at least one cation diffusion facilitator (CDF) necessary for iron transport and magnetosome membrane assembly (e.g., MamB and MamM) (205), (iii) PDZ domains that mediate protein-protein interactions (e.g., two in MamE and one in MamP) (174,199,206), (iv) a LemA domain (MamQ) whose function is uncertain (174,199) (207), and (vi) one or two actin-like domains (MamK) involved in magnetosome chain assembly and its positioning inside the cell (164,173). There are also proteins with different multiple domains, such as the protease MamE, which contains trypsin, magnetochrome, and PDZ domains, or MamO, which contains one trypsin domain and one TauE domain (174,199).…”

Physiological origin of biogenic magnetic nanoparticles in health and disease: from bacteria to humans

“…1,11,12 The loss of the magnetosome island leads to a nonmagnetic phenotype of MTB demonstrating its key role in the biogenesis of BMNPs. 11,97,[109][110][111][112][113] Figure 5 shows the participation of proteins of a magnetosome island in the biomineralization of BMNPs in a magnetotactic bacterium. The magnetosome vesicle represents a unique pool providing favorable chemical surroundings for magnetite crystal growth.…”