Surface-layer glycoproteins: an example for the diversity of bacterial glycosylation with promising impacts on nanobiotechnology.
Bacterial cell surface layers, referred to simply as S-layers, have been described for all major phylogenetic groups of bacteria, which may indicate their pivotal role for a bacterium in its natural habitat. They have the unique ability to assemble into two-dimensional crystalline arrays that completely cover the bacterial cells. Glycosylation represents the most frequent modification of S-layer proteins. S-layer glycoproteins constitute a class of glycoconjugates first isolated in the mid-1970s, but S-layer glycoprotein research is still being regarded as an "exotic field of glycobiology," possibly because of its "noneukaryotic" character. Extensive work over the past 30 years provided evidence of an enormous diversity of S-layer glycoproteins that have been created in nature over 3 billion years of prokaryotic evolution. These glycoconjugates are substantially different from eukaryotic glycoproteins, with regard to both composition and structure; nevertheless, some general structural concepts may be deduced. The awareness of the high application potential of S-layer glycoproteins, especially in combination with their intrinsic cell surface display feature, in the field of modern nanobiotechnology as a base for glycoengineering has recently led to the investigation of the S-layer protein glycosylation process at the molecular level, which has lagged behind the structural studies due to the lack of suitable molecular tools. From that work an even more interesting picture of this class of glycoconjugates is emerging. The availability of purified enzymes from S-layer glycan biosynthesis pathways exhibiting increased stabilities and/or rare sugar specificities in conjunction with preliminary genomic data on S-layer glycan biosynthesis clusters will pave the way for the rational design of S-layer neoglycoproteins.