Assembly, Spatial Distribution, and Secretion Activity of the Curlin Secretion Lipoprotein, CsgG.

Abstract

Escherichia coli assembles functional amyloid fibers called curli. Produced by many Enterobacteriaceae spp., curli fibers are associated with biofilm formation, host cell adhesion and invasion, and immune system activation. CsgB nucleates the major curli subunit protein, CsgA, into a self-propagating amyloid fiber on the cell surface. CsgA and CsgB cell surface association and subsequent fiber polymerization are efficient processes, yet little is known about how the subunits reach and become positioned on the surface. I have characterized the role of CsgG in curli subunit secretion across the outer membrane, and found that CsgG is the central molecule of the curli secretion and assembly complex. CsgG formed an oligomeric complex in the outer membrane that interacted with at least three other csg encoded proteins. I found that the CsgG lipoprotein spanned the outer membrane, was sufficient for curlin transport across the outer membrane, and that a specific signal peptide on the N-terminus of the major fiber subunit directed protein secretion via the CsgG secretion apparatus. I also discovered that CsgG was non-uniformly distributed on the surface of curli-producing cells, and that the assembly, spatial organization, and secretion activity of CsgG was modified by other components of the curli assembly machine. One of the other curli assembly proteins, CsgE, was found to specifically function in the secretion pathway and to modulate both the secretion activity of CsgG and to act directly on CsgA to prevent self-polymerization. These results suggest a model where secretion and fiber assembly are tightly coupled and highly ordered processes.