Translocase and major signal peptidase malfunctions affect aerial mycelium formation in Streptomyces lividans

The Cellular Mechanisms that Ensure an Efficient Secretion in Streptomyces

Gram-positive soil bacteria included in the genus Streptomyces produce a large variety of secondary metabolites in addition to extracellular hydrolytic enzymes. From the industrial and commercial viewpoints, the S. lividans strain has generated greater interest as a host bacterium for the overproduction of homologous and heterologous hydrolytic enzymes as an industrial application, which has considerably increased scientific interest in the characterization of secretion routes in this bacterium. This review will focus on the secretion machinery in S. lividans.

Protein Secretion in Gram-Positive Bacteria: From Multiple Pathways to Biotechnology

A number of Gram-positive bacteria are important players in industry as producers of a diverse array of economically interesting metabolites and proteins. As discussed in this overview, several Gram-positive bacteria are valuable hosts for the production of heterologous proteins. In contrast to Gram-negative bacteria, proteins secreted by Gram-positive bacteria are released into the culture medium where conditions for correct folding are more appropriate, thus facilitating the isolation and purification of active proteins. Although seven different protein secretion pathways have been identified in Gram-positive bacteria, the majority of heterologous proteins are produced via the general secretion or Sec pathway. Not all proteins are equally well secreted, because heterologous protein production often faces bottlenecks including hampered secretion, susceptibility to proteases, secretion stress, and metabolic burden. These bottlenecks are associated with reduced yields leading to non-marketable products. In this chapter, besides a general overview of the different protein secretion pathways, possible hurdles that may hinder efficient protein secretion are described and attempts to improve yield are discussed including modification of components of the Sec pathway. Attention is also paid to omics-based approaches that may offer a more rational approach to optimize production of heterologous proteins.

Exploring the Feasibility of the Sec Route to Secrete Proteins Using the Tat Route in Streptomyces lividans

Streptomyces lividans uses mainly two pathways to target secretory proteins to the cytoplasmic membrane. The major pathway (Sec pathway) transports pre-proteins using the signal recognition particle, and the minor Tat pathway is responsible for the secretion using a folded conformation of a relatively low number of proteins. The signal peptides of the Sec-dependent alpha-amylase and the Tat-dependent agarase were interchanged and fused in-frame to the corresponding mature part of the other enzyme. Alpha-amylase was unable to use the Tat route when fused to the agarase signal peptide, while agarase used the Sec route when it was targeted by the alpha-amylase signal peptide. In addition to the signal peptide some yet unidentified parts of the secreted proteins may play a role in selecting the secretory route. Structure predictions for the Tat- and Sec-dependent proteins suggest that less structured proteins are more likely to be candidates for the Tat route.