Selenite uptake by outer membrane porin ExtI and its involvement in the subcellular localization of rhodanese-like lipoprotein ExtH in Geobacter sulfurreducens.

Selenite reduction is a key step in the biogeochemical cycle of selenium-an essential trace element for life. A variety of bacteria can transform selenite into elemental selenium nanoparticles on the cell surface via anaerobic respiration or detoxification processes. However, the proteins associated with the uptake of selenite for these processes are poorly understood. In this study, we investigated the role of an outer membrane porin-like protein, ExtI, in selenite permeation in Geobacter sulfurreducens. We demonstrated that selenite uptake and selenium nanoparticle formation were impaired in an extI-deficient strain. A putative rhodanese-like lipoprotein is encoded by an extH gene located immediately upstream of extI in the genome. We showed that ExtH is translocated into inner and outer membranes and that extI deficiency exclusively affects the localization of ExtH in the outer membrane. Coelution of ExtI and ExtH during gel filtration analysis of the outer membrane fraction of wild-type cells suggests a direct protein-protein interaction between them. Taken together, these results lead us to propose a physiological role for ExtI as a selenite channel associated with ExtH in the outer membrane.