Nature's solution to the iron problem: Mechanisms of iron management in ferritins

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Iron is essential for virtually all forms of life, playing central roles in many of the reactions on which life depends. Although this metal is highly abundant in the earth's crust, it is largely 'locked up' in minerals that are highly insoluble and this severely limits its availability to living organisms. The scarcity of iron has led to the evolution in microbes of many ingenious mechanisms to obtain sufficient quantities to sustain growth. These include the secretion and re-absorption of small organic molecules that can bind iron very tightly and therefore hoover up what iron there is. Pathogenic bacteria can effectively steal iron from iron-containing proteins in their hosts, and the success of such mechanisms is a key determinant of whether or not a successful infection is established. Another important aspect of how cells overcome the iron problem, is their ability to store iron. This is achieved through a remarkable family of proteins known as ferritins. These can be thought of as being football-like molecules with a hollow centre, inside which thousands of iron atoms can be stored in the form of an iron mineral - one that would form insoluble precipititates were it not for the solubilising effect of the protein coat. The storage of iron serves two important functions. Firstly, it enables microbes to draw on reserves when iron in the immediate environment becomes particularly low, and secondly, it overcomes the potential toxicity of iron that results from the very properties that make it useful to life: without proper control, iron can lead to the generation of reactive oxygen species that can cause severe cellular damage. In this research we propose to better understand how two particular ferritins from two very different microbes take up iron and orchestrate the deposition of their iron mineral cargo, and how they subsequently release it. One of these is from the organism Escherichia coli - a bacterium that has for decades been a workhorse for understanding cellular processes, but which can also be pathogenic. The other ferritin is one that was very recently identified in phytoplankton. These organisms are responsible for a large proportion of the primary production of organic molecules from carbon dioxide in the oceans. Furthermore, they also form spectacular blooms in areas of temporary nutrient sufficiency. In many areas, the growth limiting nutrient is iron and it is now known that the presence of ferritin in these phytoplankton is crucial for their ability to utilise temporarily available iron. This research will lead to a significant advance in our understanding of iron cycling processes and of the relationship between structural and functional properties of ferritins. This will also have more general impact in understanding the reactivities of iron proteins and in protein-driven biomineralisation processes.