Surface layer (S-layer) proteins are one of the most
commonly observed cell envelope components in both Archaea and Bacteria. It has
versatile functions and holds considerable application potential in
biotechnology. Bifidobacteria are representative probiotics conferring health
promoting properties. However, there is little study of S-layer in
bifidobacteria yet. The distribution and characteristics of S-layer in
bifidobacteria are unknown. In this study, search for S-layer protein in the
identical protein groups in NCBI yielded 49 hits belonging to bifidobacteria.
These proteins were annotated as either “S-layer (domain) protein” or “putative
S-layer (y) domain protein” that distributed among 26 species of Bifidobacterium genus. Multiple
alignments suggest S-layer proteins are relatively conservative. Phylogenetic
analysis of 24 S-layer (domain) protein sequences groups them into three
distinct clusters, with the majority species in Cluster-2. S-layer (domain)
protein has a universe motif DUF4381, though its function is unknown.
Meanwhile, two other motifs CARDB and EphA2_TM involved in cell adhesion and
cell signaling respectively, presented in most S-layer (domain) protein in bifidobacteria.
All S-layer proteins have a typical N-terminal Sec-dependent signal peptide and
a C-terminal trans-membrane region. Homological modeling of representative
S-layer proteins from each cluster revealed a few unique structural features.All representative S-layer proteins have a plenty of β-meander motif that exclusively composed by β-barrel structural architectures linked together by hairpin loops.
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