Type V secretion: from biogenesis to biotechnology.

The two membranes of Gram-negative bacteria contain protein machines that have a general function in their assembly. To interact with the extra-cellular milieu, Gram-negatives target proteins to their cell surface and beyond. Many specialized secretion systems have evolved with dedicated translocation machines that either span the entire cell envelope or localize to the outer membrane. The latter act in concert with inner-membrane transport systems (i.e. Sec or Tat). Secretion via the Type V secretion system follows a two-step mechanism that appears relatively simple. Proteins secreted via this pathway are important for the Gram-negative life-style, either as virulence factors for pathogens or by contributing to the survival of non-invasive environmental species. Furthermore, this system appears well suited for the secretion of biotechnologically relevant proteins. In this review we focus on the biogenesis and application of two Type V subtypes, the autotransporters and two-partner secretion (TPS) systems. For translocation across the outer membrane the autotransporters require the assistance of the Bam complex that also plays a generic role in the assembly of outer membrane proteins. The TPS systems do use a dedicated translocator, but this protein shows resemblance to BamA, the major component of the Bam complex. Interestingly, both the mechanistic and more applied studies on these systems have provided a better understanding of the secretion mechanism and the biogenesis of outer membrane proteins. This article is part of a Special Issue entitled: Protein trafficking and secretion in bacteria. Guest Editors: Anastassios Economou and Ross Dalbey.