Nanoporous S-Layer Protein Lattices. A Biological Ion Gate with Calcium Selectivity

Abstract

S-layer protein lattice fragments isolated from the bacterium Deinococcus radiodurans were adsorbed onto chemically modified silicon substrates to produce nanoporous membranes containing uniformly sized pores with dimensions between 2 and 7 nm. The gating characteristics of the nanoporous membranes toward various ionic species were evaluated using electrochemical impedance spectroscopy. Our results reveal that the immobilized S-layers undergo a strong interaction with cations, especially Ca2+ ions. We observed for the first time ionic currents flowing through the nanoporous regions of the protein, pointing to an ion transport mechanism of gating. Our observations suggest that ion transport is mainly due to the presence of an electrical gradient inside the pores. The origin of the observed selectivity is discussed.