A Trimeric Lipoprotein assists in Trimeric Autotransporter Biogenesis in Enterobacteria [Protein Structure and Folding]

December 25th, 2013 by Grin, I., Hartmann, M. D., Sauer, G., Hernandez Alvarez, B., Schutz, M., Madlung, J., Macek, B., Felipe–Lopez, A., Hensel, M., Lupas, A., Linke, D.

Trimeric Autotransporter Adhesins (TAAs) are important virulence factors of many gram-negative bacterial pathogens. TAAs form fibrous, adhesive structures on the bacterial cell surface. Their N-terminal, extracellular domains are exported through a C-terminal membrane pore; the insertion of the pore domain into the bacterial outer membrane follows the rules of beta-barrel transmembrane protein biogenesis and is dependent on the essential Bam complex. We have recently described the full fiber structure of SadA, a TAA of unknown function in Salmonella and other enterobacteria. In this work, we describe the structure and function of SadB, a small inner membrane lipoprotein. The sadB gene is located in an operon with sadA; orthologous operons are only found in enterobacteria, while other TAAs are not typically associated with lipoproteins. Strikingly, SadB is also a trimer, and its co-expression with SadA has a direct influence on SadA structural integrity. This is the first report of a specific export factor of a TAA, suggesting that at least in some cases, TAA autotransport is assisted by additional periplasmic proteins.