Structural and Functional Insights into the Pilotin-Secretin Complex of the Type II Secretion System

Gram-negative bacteria secrete virulence factors and assemble fibre structures on their cell surface using specialized secretion systems. Three of these, T2SS, T3SS and T4PS, are characterized by large outer membrane channels formed by proteins called secretins. Usually, a cognate lipoprotein pilot is essential for the assembly of the secretin in the outer membrane. The structures of the pilotins of the T3SS and T4PS have been described. However in the T2SS, the molecular mechanism of this process is poorly understood and its structural basis is unknown. Here we report the crystal structure of the pilotin of the T2SS that comprises an arrangement of four α-helices profoundly different from previously solved pilotins from the T3SS and T4P and known four α-helix bundles. The architecture can be described as the insertion of one α-helical hairpin into a second open α-helical hairpin with bent final helix. NMR, CD and fluorescence spectroscopy show that the pilotin binds tightly to 18 residues close to the C-terminus of the secretin. These residues, unstructured before binding to the pilotin, become helical on binding. Data collected from crystals of the complex suggests how the secretin peptide binds to the pilotin and further experiments confirm the importance of these C-terminal residues in vivo

SdiA, an N-Acylhomoserine Lactone Receptor, Becomes Active during the Transit of Salmonella enterica through the Gastrointestinal Tract of Turtles

encode a LuxR-type AHL receptor, SdiA, they cannot synthesize AHLs. In vitro, it is known that SdiA can detect AHLs produced by other bacterial species..We conclude that the normal gastrointestinal microbiota of most animal species do not produce AHLs of the correct type, in an appropriate location, or in sufficient quantities to activate SdiA. However, the results obtained with turtles represent the first demonstration of SdiA activity in animals

Teaching: Natural or Cultural?

In this chapter I argue that teaching, as we now understand the term, is historically and cross-culturally very rare. It appears to be unnecessary to transmit culture or to socialize children. Children are, on the other hand, primed by evolution to be avid observers, imitators, players and helpers—roles that reveal the profoundly autonomous and self-directed nature of culture acquisition (Lancy in press a). And yet, teaching is ubiquitous throughout the modern world—at least among the middle to upper class segment of the population. This ubiquity has led numerous scholars to argue for the universality and uniqueness of teaching as a characteristically human behavior. The theme of this chapter is that this proposition is unsustainable. Teaching is largely a result of recent cultural changes and the emergence of modern economies, not evolution

Domain structure of secretin PulD revealed by limited proteolysis and electron microscopy

Secretins, a superfamily of multimeric outer membrane proteins, mediate the transport of large macromolecules across the outer membrane of Gram-negative bacteria. Limited proteolysis of secretin PulD from the Klebsiella oxytoca pullulanase secretion pathway showed that it consists of an N-terminal domain and a protease-resistant C-terminal domain that remains multimeric after proteolysis. The stable C-terminal domain starts just before the region in PulD that is highly conserved in the secretin superfamily and apparently lacks the region at the C-terminal end to which the secretin-specific pilot protein PulS binds. Electron microscopy showed that the stable fragment produced by proteolysis is composed of two stacked rings that encircle a central channel and that it lacks the peripheral radial spokes that are seen in the native complex. Moreover, the electron microscopic images suggest that the N-terminal domain folds back into the large cavity of the channel that is formed by the C-terminal domain of the native complex, thereby occluding the channel, consistent with previous electrophysiological studies showing that the channel is normally closed