Four-fold channels are involved in iron diffusion into the inner cavity of plant ferritin

From an evolutionary point of view, plant and animal ferritins arose from a common ancestor, but plant ferritin exhibits different features as compared with the animal analogue. One major difference is that the 4-fold channels naturally occurring in plant ferritin are hydrophilic, whereas the 4-fold channels in animal ferritin are hydrophobic. Prior to this study, however, the function of the 4-fold channels in oxidative deposition of iron in phytoferritin remained unknown. To elucidate the role of the 4-fold channels in iron oxidative deposition in ferritin, three mutants of recombinant soybean seed H-2 ferritin (rH-2) were prepared by site-directed mutagenesis, which contained H193A/H197A, a 4-fold channel mutant, E165I/E167A/E171A, a 3-fold channel mutant, and E165I/E167A/E171A/H193A/H197A, where both 3- and 4-channels were mutated. Stopped-flow, electrode oximetry, and transmission electron microscopy (TEM) results showed that H193A/H197A and E165I/E167A/E171A exhibited a similar catalyzing activity of iron oxidation with each other, but a pronounced low activity compared to rH-2, demonstrating that both the 4-fold and 3-fold hydrophilic channels are necessary for iron diffusion in ferritin, followed by oxidation. Indeed, among all tested ferritin, the catalyzing activity of E165I/E167A/E171A/H193A/H197A was weakest because its 3- and 4- fold channels were blocked. These findings advance our understanding of the function of 4-fold channels of plant ferritin and the relationship of the structure and function of ferritin.