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Purification of the M. magneticum Strain AMB-1 Magnetosome Associated Protein MamAΔ41
Published on: March 25, 2010
The Magnetosome Protein, Mms6 from Magnetospirillum magneticum Strain AMB-1, Is a Lipid-Activated Ferric Reductase
Dilini Singappuli-Arachchige1,2, Shuren Feng1,3, Lijun Wang1,3
1Ames Laboratory, U.S. Department of Energy, Ames, IA 50011, USA.
Abstract:
Magnetosomes of magnetotactic bacteria consist of magnetic nanocrystals with defined morphologies enclosed in vesicles originated from cytoplasmic membrane invaginations. Although many proteins are involved in creating magnetosomes, a single magnetosome protein, Mms6 from Magnetospirillum magneticum strain AMB-1, can direct the crystallization of magnetite nanoparticles in vitro. The in vivo role of Mms6 in magnetosome formation is debated, and the observation that Mms6 binds Fe3+ more tightly than Fe2+ raises the question of how, in a magnetosome environment dominated by Fe3+, Mms6 promotes the crystallization of magnetite, which contains both Fe3+ and Fe2+. Here we show that Mms6 is a ferric reductase that reduces Fe3+ to Fe2+ using NADH and FAD as electron donor and cofactor, respectively. Reductase activity is elevated when Mms6 is integrated into either liposomes or bicelles. Analysis of Mms6 mutants suggests that the C-terminal domain binds iron and the N-terminal domain contains the catalytic site. Although Mms6 forms multimers that involve C-terminal and N-terminal domain interactions, a fusion protein with ubiquitin remains a monomer and displays reductase activity, which suggests that the catalytic site is fully in the monomer. However, the quaternary structure of Mms6 appears to alter the iron binding characteristics of the C-terminal domain. These results are consistent with a hypothesis that Mms6, a membrane protein, promotes the formation of magnetite in vivo by a mechanism that involves reducing iron.
Insights
Magnetospirillum magneticum Mms6 protein acts as a ferric reductase, converting Fe3+ to Fe2+ using NADH and FAD. This iron reduction mechanism is crucial for in vivo magnetite biomineralization in magnetotactic bacteria.
Area of Science:
- Biochemistry
- Microbiology
- Biomineralization
Background:
- Magnetosomes are vesicles containing magnetic nanocrystals formed by magnetotactic bacteria.
- Mms6 protein from Magnetospirillum magneticum AMB-1 can direct magnetite crystallization in vitro.
- The in vivo function of Mms6 and its role in magnetite formation, particularly its high affinity for Fe3+, remain unclear.
Purpose of the Study:
- To investigate the in vivo role of Mms6 in magnetosome formation.
- To elucidate the mechanism by which Mms6 promotes magnetite crystallization despite the Fe3+-rich environment.
Main Methods:
- Biochemical assays to determine Mms6 reductase activity using NADH and FAD.
- In vitro studies using liposomes and bicelles to assess Mms6 activity in membrane-like environments.
- Analysis of Mms6 mutants to identify functional domains (N-terminal catalytic, C-terminal iron-binding).
- Investigating the effect of Mms6 quaternary structure (multimers vs. monomer) on its activity.
Main Results:
- Mms6 functions as a ferric reductase, reducing Fe3+ to Fe2+ with NADH and FAD.
- Mms6 reductase activity is enhanced when integrated into liposomes or bicelles.
- Mutant analysis indicates distinct N-terminal (catalytic) and C-terminal (iron-binding) domains.
- Monomeric Mms6 retains reductase activity, suggesting the catalytic site is within the monomer.
- Quaternary structure influences the iron-binding characteristics of the C-terminal domain.
Conclusions:
- Mms6 is a ferric reductase essential for magnetite biomineralization in magnetotactic bacteria.
- The reduction of Fe3+ to Fe2+ by Mms6 is a key step in promoting magnetite formation in vivo.
- Mms6's membrane association and its reductase activity are critical for its function in magnetosome development.
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