Improving extracellular protein production in Escherichia coli by overexpressing D,D-carboxypeptidase to perturb peptidoglycan network synthesis and structure.

Most recombinant proteins in Escherichia coli are not efficiently secreted to the extracellular space. Structural stabilisation of the cell wall is essential for extracellular protein production in E. coli, for which D,D-carboxypeptidases are essential. Herein, we perturbed the peptidoglycan structure of the E. coli cell wall by overexpressing D,D-carboxypeptidase genes dacA or dacB, and investigated the effect on extracellular protein production. Overexpression of dacA or dacB promoted the accumulation of intracellular soluble peptidoglycan, altered cell morphology (shape and size) and led to the formation of transparent globular structures in E. coli cells. Compared with controls (CK), extracellular production of recombinant green fluorescent protein (GFP) was increased by 1.7- and 2.3-fold upon overexpression of dacA and dacB, respectively. Similarly, extracellular production of recombinant amylase and α-galactosidase was increased by 4.5- and 2.8-fold, respectively, upon overexpression of dacA, and by 11.9- and 2.5-fold, respectively, upon overexpression of dacB. Overexpression of dacA or dacB enhanced both the outer and inner membrane permeability of E. coli. This cell wall engineering strategy opens up a new direction for enhancing extracellular protein and chemical production in E. coli.