Insights into the use of specific metal binding of self-assembling S-layer proteins

Most bacteria and all archaea possess as outermost cell envelope surface-layer (S-layer) proteins. These self-assembling proteins form nanostructured lattices with different symmetries, provide regular arranged pores with defined size and possess different kinds of regular arranged functional groups. The formation of stable and functional S-layer lattices via self-assembly on the cell surface, on different technical surfaces as well as interfaces is a dynamic and complex process. Despite the fact that S-layer proteins have been investigated for over 30 years, the full reaction cascade of self-assembly, which includes the role of different bivalent cations such as Ca2+ and Mg2+, are still not fully understood.
Furthermore, S-layers have a number of important intrinsic properties, e.g. they provide cellular wall protection, mediate selective exchange of molecules and therefore function as molecular sieves. Interestingly, S-layers from bacterial isolates recovered from heavy metal contaminated environments have outstanding metal binding properties and are highly stable. They show potential for selective binding of several metals some of them with high affinity. Therefore, three aspects of the metal-interactions with S-layer proteins must be taken into account.
First, S-layers possess different functionalities, e.g. carboxyl-, phosphoryl, hydroxyl groups, binding toxic metals and metalloids, like U and As, unspecifically and by this hinder them to enter the interior of prokaryotic cells. This interaction process is strongly driven by pH-value as the functionalities need to be deprotonated. Second, precious metals like Au and Pd are likewise bound unspecifically to functional groups, but presumably covalently making the binding irreversible unless the S-layer protein is destroyed completely.
Third, some metals are needed for native protein folding of the S-layer protein monomer, self-assembly, and the formation of highly-ordered lattices. These particular metals are bivalent cations such as Ca2+. As known from titration experiments, certain S-layer proteins bind Ca2+ specifically, thereby forming very stable complexes. There are at least two different binding sites for these bivalent cations showing different binding affinities. Important is that these binding sites not only allow selective binding of calcium, but also of chemical-equal elements including the trivalent lanthanides (Eu3+, Tb3+), possessing comparable ionic radii. This was proven by titration and laser fluorescence spectroscopic experiments.
This study shows that the intrinsic properties and physiological functions of the S-layer proteins build the base for its selective metal binding behavior and its potential for fabrication of biohybrid materials. So by combining S-layers with a layer-by-layer technique different materials can be furnished with coatings. The produced biohybrid materials can be directly used as selective metal filter material for the removal or recovery of strategic relevant metals using pH-value as regulating parameter for selective metal binding and also conceivably release.