Interactions of the native and engineered S-layer of Bacillus sphaericus JG-A12 with U, Pd, and other metals

The cells of the uranium mining waste pile isolate B. sphaericus JG-A12 are enveloped by a highly ordered paracrystalline protein surface layer (S-layer) with a square lattice structure (1). The protein consists of 1238 amino acids, contains three N terminal S-layer homologous (SLH) domains and is, in contrast to the other Bacillus S-layers studied up to date, phosphorylated. A comparative analysis of the amino acid sequence of the JG-A12 S-layer protein with those deposited in the GeneBank demonstrated that the SLH domains of JG-A12 possess an unique structure. However, particular parts of the central and of the C-terminal domains of the protein share high identity with some of the known B. sphaericus S-layers (2). The latter is an indication that the JG-A12 S-layer protein gene was subjected to late evolutionary events via lateral genetic transfer and rearrangements.
The interactions of the purified and re-crystallized S-layer with U (VI) and Pd (II) were studied by using x-ray and infrared spectroscopic analyses. The results of these analyses show that U(VI) is coordinated to both phosphate and carboxyl groups of the S-Layer whereas Pd(II) was bound mainly by carboxyl groups. This is in accordance to our finding that the protein is phosphorylated and also that aspartate and glutamate rich stretches are distributed in its C-terminal domain which is supposed to build the S-layer lattice pores. The latter are involved in the complexation and also in the deposition of the metals. The infrared spectroscopic analysis indicated in addition that the deposition of the Pd complexes stabilizes the structure of the protein.
In our former work the S-layer of B. sphaericus JG-A12 was successfully used for the construction of sol-gel Si02 bio-ceramics for remediation of uranium contaminated waters (3). In addition, the S-layer of another B. sphaericus strain, which is closely related to the JG-A12, was used as a template for deposition of metallic Pd nanoclusters (4). Such nanoclusters are of great importance for the development of novel catalysts and biosensors for the industry.
In this work we apply genetic engineering of the S-layer of B. sphaericus JG-A12 in order to extend its capability to interact with other metals of industrial interest.