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Key carboxylate residues for iron transit through the prokaryotic ferritin SynFtn

DOI: 10.1099/mic.0.001105 DOI Help

Authors: Justin M. Bradley (University of East Anglia) , Joshua Fair (University of East Anglia) , Andrew M. Hemmings (University of East Anglia) , Nick Le Brun (University of East Anglia)
Co-authored by industrial partner: No

Type: Journal Paper
Journal: Microbiology , VOL 167

State: Published (Approved)
Published: November 2021
Diamond Proposal Number(s): 25108

Open Access Open Access

Abstract: Ferritins are proteins forming 24meric rhombic dodecahedral cages that play a key role in iron storage and detoxification in all cell types. Their function requires the transport of Fe2+ from the exterior of the protein to buried di-iron catalytic sites, known as ferroxidase centres, where Fe2+ is oxidized to form Fe3+-oxo precursors of the ferritin mineral core. The route of iron transit through animal ferritins is well understood: the Fe2+ substrate enters the protein via channels at the threefold axes and conserved carboxylates on the inner surface of the protein cage have been shown to contribute to transient binding sites that guide Fe2+ to the ferroxidase centres. The routes of iron transit through prokaryotic ferritins are less well studied but for some, at least, there is evidence that channels at the twofold axes are the major route for Fe2+ uptake. SynFtn, isolated from the cyanobacterium Synechococcus CC9311, is an atypical prokaryotic ferritin that was recently shown to take up Fe2+ via its threefold channels. However, the transfer site carboxylate residues conserved in animal ferritins are absent, meaning that the route taken from the site of iron entry into SynFtn to the catalytic centre is yet to be defined. Here, we report the use of a combination of site-directed mutagenesis, absorbance-monitored activity assays and protein crystallography to probe the effect of substitution of two residues potentially involved in this pathway. Both Glu141 and Asp65 play a role in guiding the Fe2+ substrate to the ferroxidase centre. In the absence of Asp65, routes for Fe2+ to, and Fe3+ exit from, the ferroxidase centre are affected resulting in inefficient formation of the mineral core. These observations further define the iron transit route in what may be the first characterized example of a new class of ferritins peculiar to cyanobacteria.

Journal Keywords: ferritin; iron; transport; oxidation; storage

Subject Areas: Biology and Bio-materials


Instruments: I03-Macromolecular Crystallography

Added On: 01/12/2021 10:32

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mic001105.pdf

Discipline Tags:

Life Sciences & Biotech Structural biology

Technical Tags:

Diffraction Macromolecular Crystallography (MX)