Co-expression of holin gene improves heterologous cellulase secretion and surface display by lactic acid bacteria Lactococcus cremoris

Genetically modified Lactococcus cremoris strains producing heterologous cellulases emerge as promising candidates for cost-effective bioprocessing of plant biowaste into high-value organic compounds. A key challenge in enabling efficient cellulose degradation by L. cremoris remains the effective secretion of heterologous cellulases. A potential solution would be to use genetic engineering to enhance the permeabilization of bacterial cell membrane and cell wall to facilitate the release of cellulases. This could be achieved by the co-expression of cellulase genes with genes encoding the prophage lytic proteins such as holin and endolysin. Co-expression of genes encoding the prophage protein holin and heterologous cellulases Cel5I, Cel9A and Cel5H in L. cremoris significantly improved secretion of Cel5I (up to 15-fold) and Cel9A (at least 3-fold), as well as surface display of Cel5I, Cel5H and Cel9A. Comparable levels of secretion and surface display of cellulases were observed regardless of whether the holin gene was co-expressed from a plasmid or inserted into the genome. In contrast, co-expression of gene encoding the prophage protein endolysin did not improve secretion of cellulase even when produced simultaneously with its partner protein holin. Co-production of holin with heterologous cellulases appears as a promising strategy for improving cellulolytic capability of L. cremoris by increasing their secretion. This might advance the development of consolidated bioprocesses involving lactic acid bacteria for valorisation of cellulosic material. Additionally, the same approach might likewise be employed to enhance the secretion and surface display of other heterologous recombinant proteins.