Self-associating autotransporters, SAATs: functional and structural similarities

The autotransporter family of translocated proteins in Gram-negative bacteria all contain three structural motifs, a signal sequence, a passenger domain and a translocator domain. The autotransporters constitute a highly versatile group of proteins with respect to function, which accords with the widespread presence of these proteins. The group encompasses many important virulence factors. In Escherichia coli, a subgroup of autotransporter proteins consists of the TibA adhesin/invasin associated with some enterotoxigenic E. coli, the AIDA adhesin from diarrhea-causing E. coli and finally, the Ag43 autoaggregation factor found in the majority of E. coli strains. The three proteins exhibit approximately 25% identity at the sequence level, and are quite different with respect to size, glycosylation and processing. Nevertheless, they share some important properties: all are self-associating proteins that cause bacterial aggregation. They can also interact with each other via heterologous interactions to cause formation of mixed bacterial aggregates. Furthermore, these proteins enhance biofilm formation. Based on these properties we propose to classify them together in a group termed SAATs: self-associating autotransporters.