| Summary: | Lysin motif (LysM) domains are found in many bacterial peptidoglycan hydrolases. They can bind non-covalently to peptidoglycan and have been employed to display heterologous proteins on the bacterial cell surface. In this study, we aimed to use a single LysM domain derived from a putative extracellular transglycosylase Lp_3014 of <i>Lactobacillus plantarum</i> WCFS1 to display two different lactobacillal β-galactosidases, the heterodimeric LacLM-type from <i>Lactobacillus reuteri</i> and the homodimeric LacZ-type from <i>Lactobacillus delbrueckii</i> subsp. <i>bulgaricus</i>, on the cell surface of different <i>Lactobacillus</i> spp. The β-galactosidases were fused with the LysM domain and the fusion proteins, LysM-LacLMLreu and LysM-LacZLbul, were successfully expressed in <i>Escherichia coli</i> and subsequently displayed on the cell surface of <i>L. plantarum</i> WCFS1. β-Galactosidase activities obtained for <i>L. plantarum</i> displaying cells were 179 and 1153 U per g dry cell weight, or the amounts of active surface-anchored β-galactosidase were 0.99 and 4.61 mg per g dry cell weight for LysM-LacLMLreu and LysM-LacZLbul, respectively. LysM-LacZLbul was also displayed on the cell surface of other <i>Lactobacillus</i> spp. including <i>L. delbrueckii</i> subsp. <i>bulgaricus</i>, <i>L. casei</i> and <i>L. helveticus</i>, however <i>L. plantarum</i> is shown to be the best among <i>Lactobacillus</i> spp. tested for surface display of fusion LysM-LacZLbul, both with respect to the immobilization yield as well as the amount of active surface-anchored enzyme. The immobilized fusion LysM-β-galactosidases are catalytically efficient and can be reused for several repeated rounds of lactose conversion. This approach, with the β-galactosidases being displayed on the cell surface of non-genetically modified food-grade organisms, shows potential for applications of these immobilized enzymes in the synthesis of prebiotic galacto-oligosaccharides.
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