Surface display of heterologous proteins in Bacillus thuringiensis using a peptidoglycan hydrolase anchor

Previous studies have revealed that the lysin motif (LysM) domains of bacterial cell wall-degrading enzymes are able to bind to peptidoglycan moieties of the cell wall. This suggests an approach for a cell surface display system in Gram-positive bacteria using a LysM-containing protein as the anchoring motif. In this study, we developed a new surface display system in B. thuringiensis using a LysM-containing peptidoglycan hydrolase, endo-beta-N-acetylglucosaminidase (Mbg), as the anchor protein.

Homology searching in the B. thuringiensis YBT-1520 genome revealed a putative peptidoglycan hydrolase gene. The encoded protein, Mbg, exhibited substantial cell-wall binding capacity. The deduced amino acid sequence of Mbg was structurally distinguished as an N-terminal domain with two tandemly aligned LysMs and a C-terminal catalytic domain. A GFP-fusion protein was expressed and used to verify the surface localization by Western blot, flow cytometry, protease accessibility, SDS sensitivity, immunofluorescence, and electron microscopy assays. Low-level constitutive expression of Mbg was elevated by introducing a sporulation-independent promoter of cry3Aa. Truncated Mbg domains with separate N-terminus (Mbgn), C-terminus (Mbgc), LysM1, or LysM2 were further compared for their cell-wall displaying efficiencies. The Mbgn moiety contributed to cell-wall anchoring, while LysM1 was the active domain. Two tandemly repeated Mbgns exhibited the highest display activity, while the activity of three repeated Mbgns was decreased. A heterologous bacterial multicopper oxidase (WlacD) was successfully displayed onto the surface of B. thuringiensis target cells using the optimum (Mbgn)2 anchor, without radically altering its catalytic activity.

Mbg can be a functional anchor protein to target different heterologous proteins onto the surface of B. thuringiensis cells. Since the LysM domain appears to be universal in Gram-positive bacteria, the strategy presented here could be applicable in other bacteria for developing this type of system.