Characterization of the Essential Transport Function of the AIDA-I Autotransporter and Evidence Supporting Structural Predictions

The current model for autodisplay suggests a mechanism that allows a passenger protein to be translocated across the outer membrane by coordinate action of a C-terminal β-barrel and its preceding linking region. The passenger protein, linker, and β-barrel are together termed the autotransporter, while the linker and β-barrel are here referred to as the translocation unit (TU). We characterized the minimal TU necessary for autodisplay with the adhesin-involved-in-diffuse-adherence (AIDA-I) autotransporter. The assumed β-barrel structure at the C terminus of the AIDA-I autotransporter was studied by constructing a set of seven AIDA-I–cholera toxin B subunit fusion proteins containing various portions of AIDA-I. Surface exposure of the cholera toxin B moiety was assessed by dot blot experiments and trypsin accessibility of the chimeric proteins expressed in Escherichia coli JK321 or UT5600. Export of cholera toxin B strictly depended on a complete predicted β-barrel region. The absolute necessity for export of a linking region and its influence on expression as an integral part of the TU was also demonstrated. The different electrophoretic mobilities of native and denatured chimeras indicated that the proposed β-barrel resides within the C-terminal 312 amino acids of AIDA-I. Together these data provide evidence for the predicted β-barrel structure and support our formerly proposed model of membrane topology of the AIDA-I autotransporter.