Large protein organelles form a new iron sequestration system with high storage capacity.

Tobias W Giessen,Melissa G Chambers,David C Bell,Gabriel Birrane,Benjamin J Orlando,Maofu Liao,Pamela A Silver,Jules Gardener,Andrew A Verdegaal

doi:10.7554/elife.46070

Abstract

Iron storage proteins are essential for cellular iron homeostasis and redox balance. Ferritin proteins are the major storage units for bioavailable forms of iron. Some organisms lack ferritins, and it is not known how they store iron. Encapsulins, a class of protein-based organelles, have recently been implicated in microbial iron and redox metabolism. Here, we report the structural and mechanistic characterization of a 42 nm two-component encapsulin-based iron storage compartment from Quasibacillus thermotolerans. Using cryo-electron microscopy and x-ray crystallography, we reveal the assembly principles of a thermostable T = 4 shell topology and its catalytic ferroxidase cargo and show interactions underlying cargo-shell co-assembly. This compartment has an exceptionally large iron storage capacity storing over 23,000 iron atoms. Our results reveal a new approach for survival in diverse habitats with limited or fluctuating iron availability via an iron storage system able to store 10 to 20 times more iron than ferritin.

Highlights

Iron is essential to virtually all organisms on earth
We have identified a novel type of encapsulin operon involved in iron metabolism in a range of Firmicutes we term the Iron-Mineralizing Encapsulin-Associated Firmicute (IMEF)-system (Giessen and Silver, 2017)
IMEF-systems are found in Firmicute genomes and their operon organization indicates a function in dynamic iron storage

Summary

Introduction

Iron is essential to virtually all organisms on earth. It is needed for a wide variety of catalytic and redox processes ranging from cellular energy production via oxidative phosphorylation to oxygen transport by hemoglobin (Sanchez et al, 2017). Through single-particle cryo-EM analysis, we determined the structure of the Qs IMEF encapsulin shell at an overall resolution of 3.85 A (Figure 1—figure supplement 2a and Supplementary file 2). Thin section negative stain transmission electron microscopy (TEM) of E. coli cells grown in Fe-rich (4 mM) medium and expressing the Qs IMEF core operon results in the formation of clusters of large intracellular electron-dense particles (Figure 4a and Figure 4—figure supplement 1a).

Results

Conclusion