The other types of secretion systems use alternative strategies to pass through the outer membrane that do not contain the conserved ‘secretin domain’. Many of these secretory nanomachines are of therapeutic interest owing to their roles in export of virulence factors learn more during bacterial infection. Secretins are homo-multimeric complexes that form a gated channel in the outer membrane that open to allow passage of folded proteins,

assembled multi-protein complexes, and DNA. Efforts to determine the structures of secretins by X-ray crystallography and electron microscopy were recently reviewed by Korotkov et al. (2011). The protein that multimerizes to form the secretin is typically comprised of two parts: a conserved C-terminal region containing the ‘secretin domain’ that is embedded into the outer membrane and a variable, system-specific N-terminal region. Both of these regions may interact with other components of the system as well as with the substrates to be secreted or internalized. The N-terminal region contains several different types of subdomains: (1) a N0 domain that resembles the TonB-dependent signaling receptor that may allow signal

transduction between the inner membrane and outer membrane components of the system during secretion or uptake (Larsen et al., 1999; Brillet this website et al., 2007); (2) up to three heterogeneous nuclear ribonucleoprotein K homology-like domains that may fulfill the DNA binding role of competence Bcl-w systems (Tarry et al., 2011); and (3) additional elements that have yet to be structurally characterized. Despite the similarities in the overall architecture of the proteins forming secretins, the mechanisms that control secretin assembly vary both between and within systems. This review provides an overview of the differences in the assembly requirements

of secretins. Particular focus will be given to the variability in the structure and function of pilotins and accessory proteins and their role in secretin stabilization, localization and/or assembly, their mode of interaction with the secretin-forming protein, and the effect(s) that the absence of the pilotin or accessory protein has on the secretin. Proteins involved in secretin assembly are diverse in structure, functional role, and genomic context. These differences may reflect the evolutionary divergence from an ancestral secretin by recruitment of a specific set of proteins to optimize the system for a particular function. Generally, there are two classes of ancillary proteins: (1) pilotins and (2) accessory proteins. Localization and/or assembly of secretins is the proposed function of pilotins (Table 1). Pilotins have a type II N-terminal signal sequence followed by a conserved cysteine, which allows the protein to be lipidated and transferred to the inner leaflet of the outer membrane by the Lol system (Okuda & Tokuda, 2010).