Type V Protein Secretion Pathway: the Autotransporter Story

Abstract

Gram-negative bacteria possess an outer membrane layer which constrains uptake and secretion of solutes and polypeptides. To overcome this barrier, bacteria have developed several systems for protein secretion. The type V secretion pathway encompasses the autotransporter proteins, the two-partner secretion system, and the recently described type Vc or AT-2 family of proteins. Since its discovery in the late 1980s, this family of secreted proteins has expanded continuously, due largely to the advent of the genomic age, to become the largest group of secreted proteins in gram-negative bacteria. Several of these proteins play essential roles in the pathogenesis of bacterial infections and have been characterized in detail, demonstrating a diverse array of function including the ability to condense host cell actin and to modulate apoptosis. However, most of the autotransporter proteins remain to be characterized. In light of new discoveries and controversies in this research field, this review considers the autotransporter secretion process in the context of the more general field of bacterial protein translocation and exoprotein function

A conserved stable core structure in the passenger domain β‐helix of autotransporter virulence proteins

“…5 The mechanism of AT secretion is not well understood, nor is it known if a result obtained for an individual AT protein represents a general feature of the AT mechanism, or a protein-specific trait. The denaturant-induced unfolding of the Pet AT passenger reported here is strikingly similar to the equilibrium unfolding behavior of the pertactin AT passenger.…”

“…Moreover, sequence analysis suggests that threestate folding and a C-terminal stable core structure could protein sequence consist of three parts: an N-terminal signal sequence, a central heterologous passenger domain that represents the mature, functional virulence protein, and a C-terminal porin domain. 5 The signal sequence facilitates translocation across the inner membrane. The C-terminal porin domain folds into a b-barrel across the outer membrane, where it is thought to act as a pore for the transport of the passenger domain across the OM.…”

“…The C-terminal porin domain folds into a b-barrel across the outer membrane, where it is thought to act as a pore for the transport of the passenger domain across the OM. 5 The crystal structure of the porin domain from the AT NalP revealed a pore diameter that is too small (1-2 nm) to allow passage of a folded passenger domain, suggesting the passenger domain might pass through the porin unfolded, then fold extracellularly. 6 The crystal structures of two AT passenger domains have been solved: pertactin 7 and hemoglobin protease 8 (Figure 1), as well as a portion of the VacA passenger domain.…”

“…Intriguingly, [97% of all known AT proteins are predicted to include b-helical structure. 1 There is no significant concentration of ATP in the periplasm, nor a proton gradient across the OM, 5 leaving the energetic driving force for efficient OM secretion of the AT passenger unknown. We have hypothesized that the free energy released on folding of the passenger b-helix could actively contribute to efficient secretion, or alternatively passively prevent sliding of the passenger back into the periplasm during OM secretion.…”

“…5 Two well-studied examples are hemoglobin protease (Hbp), a serine protease involved in iron scavenging for the human pathogen E. coli EB1, 13 and plasmid-encoded toxin (Pet) from E. coli 042, a pathogenic strain that causes diarrhea. 14 The crystal structure of the Pet passenger domain has not been solved, but Pet and Hbp passenger domains share 26% sequence identity plus 43% sequence similarity.…”