Mutations affecting the biogenesis of the AIDA-I autotransporter

Autotransporters are simple systems that Gram-negative bacteria employ to secrete proteins to their surfaces or into the extracellular milieu. They consist of an N-terminal passenger domain and a C-terminal domain that is thought to insert into the outer membrane and mediate the secretion of the passenger domain. Despite the apparent simplicity of these secretion systems, their mechanism of translocation is still not completely understood. To study this mechanism, we used the AIDA-I autotransporter adhesin of Escherichia coli. We introduced mutations at several sites in a junction region of the passenger domain, close to the membrane-embedded domain. We observed that the mutations dramatically affected the biogenesis of AIDA-I. The same mutations, however, did not affect the translocation of a chimeric construct where MalE, the E. coli periplasmic maltose binding protein, replaced most of the passenger domain of AIDA-I. Our results emphasize the function of this region in the biogenesis of AIDA-I and suggest that it plays its role by interacting with and/or promoting folding of native passenger domains.