Isolation, gene structure, and comparative analysis of the S-layer gene sslA of Sporosarcina ureae ATCC 13881

Targeting at the Nanoscale: A Novel S-Layer Fusion Protein Enabling Controlled Immobilization of Biotinylated Molecules

With the aim of constructing an S-layer fusion protein that combines both excellent self-assembly and specific ligand i.e., biotin binding ability, streptavidin (aa 16-133) was fused to the S-layer protein of Sporosarcina ureae ATCC 13881 (SslA) devoid of its N-terminal 341 and C-terminal 172 amino acids. The genetically engineered chimeric protein could be successfully produced in E. coli, isolated, and purified via Ni affinity chromatography. In vitro recrystallisation experiments performed with the purified chimeric protein in solution and on a silicon wafer have demonstrated that fusion of the streptavidin domain does not interfere with the self-assembling properties of the S-layer part. The chimeric protein self-assembled into multilayers. More importantly, the streptavidin domain retained its full biotin-binding ability, a fact evidenced by experiments in which biotinylated quantum dots were coupled to the fusion protein monomers and adsorbed onto the in vitro recrystallised fusion protein template. In this way, this S-layer fusion protein can serve as a functional template for the controlled immobilization of biotinylated and biologically active molecules.

Truncation Derivatives of the S-Layer Protein of Sporosarcina ureae ATCC 13881 (SslA): Towards Elucidation of the Protein Domain Responsible for Self-Assembly

The cell surface of Sporosarcina ureae ATCC 13881 is covered by an S-layer (SslA) consisting of identical protein subunits that assemble into lattices exhibiting square symmetry. In this work the self-assembly properties of the recombinant SslA were characterised with an emphasis on the identification of protein regions responsible for self-assembly. To this end, recombinant mature SslA (aa 31-1097) and three SslA truncation derivatives (one N-terminal, one C-terminal and one CN-terminal) were produced in a heterologous expression system, isolated, purified and their properties analysed by in vitro recrystallisation experiments on a functionalised silicon wafer. As a result, recombinant mature SslA self-assembled into crystalline monolayers with lattices resembling the one of the wild-type SslA. The study identifies the central protein domain consisting of amino acids 341-925 self-sufficient for self-assembly. Neither the first 341 amino acids nor the last 172 amino acids of the protein sequence are required to self-assemble into lattices.

Cloning of the surface layer gene sllB from Bacillus sphaericus ATCC 14577 and its heterologous expression and purification

A cDNA fragment encoding the S-layer protein SllB cloned from Bacillus sphaericus ATCC 14577 was expressed on the surface of E. coli BL21 (DE3) cells and confirmed by the square lattice structure at the nanoscale level. The amplified gene fragment designed with PCR primers from a specified reference sequence (GenBank accession no. AJ849550) showed a high degree of sequence identity with the known sequences for S-layer protein. The best alignment scores were seen in B. sphaericus strains JG-A12 and NCTC9602, which code for a pre-form protein with a predicted cleavage site located between the two alanine residues 31 and 32. After this signal peptide sequence was removed, the mature protein had a molecular mass of 116.2613 kDa and a theoretical pI of 5.40. Further bioinformatic analysis revealed three S-layer homology (SLH) domains in the N-terminus of the mature protein, positioned at the 1–61, 63–128 and 137–197 residues. The mature S-layer protein was composed of alpha helices (24.86%), extended strands (27.01%), and rich random coils (48.13%). Bioinformatics-driven characterization of SllB may provide scientific evidence for further application of this gene in the fields of nanobiotechnology and biomimetics in the future.