Heterologous protein secretion by bacillus species from the cradle to the grave

The Gram-positive bacterium Bacillus subtilis and some of its close relatives are widely used for the industrial production of enzymes for the detergents, food, and beverage industries. The choice of these organisms is based almost exclusively on the high capacity of their secretion systems that are, under the right conditions, able to secrete proteins at grams per liter concentrations. In contrast, there are relatively few examples of Bacillus species being used for the cytoplasmic production of proteins. The range of proteins that are capable of high-level production and secretion is limited by a combination of characteristics of both the target protein and the host bacterium. The secretion pathway includes checkpoints that are designed to validate the authenticity of pathway substrates. Although many of these checkpoints are known, only some can be overcome by reengineering the host. As a result, the yield of heterologous protein production is extremely variable. In this review, we consider the Bacillus protein secretion pathway from the synthesis of the target protein (cradle) to its emergence at the outer surface of the complex cell wall (grave), and discuss the roles of the various checkpoints both with respect to the target protein and their role on cell homeostasis.