Charge-dependent secretion of an intrinsically disordered protein via the autotransporter pathway

Autotransporters are a large class of virulence proteins produced by Gram-negative bacteria. They contain an N-terminal extracellular (passenger) domain that folds into a C-helical structure and a C-terminal beta-barrel (beta) domain that anchors the protein to the outer membrane. Because the periplasm lacks ATP, the source of energy that drives passenger domain secretion is unknown. The prevailing model postulates that vectorial folding of the beta-helix in the extracellular space facilitates unidirectional secretion of the passenger domain. In this study we used a chimeric protein composed of the 675-residue receptor-binding domain (RD) of the Bordetella pertussis adenylate cyclase toxin CyaA fused to the C terminus of the Escherichia coli O157:H7 autotransporter EspP to test this hypothesis. The RD is a highly acidic, repetitive polypeptide that is intrinsically disordered in the absence of calcium. Surprisingly, we found that the RD moiety was efficiently secreted when it remained in an unfolded conformation. Furthermore, we found that neutralizing or reversing the charge of acidic amino acid clusters stalled translocation in the vicinity of the altered residues. These results challenge the vectorial folding model and, together with the finding that naturally occurring passenger domains are predominantly acidic, provide evidence that a net negative charge plays a significant role in driving the translocation reaction.