Assisted Secretion of a Trimeric Autotransporter Adhesin from Salmonella

Abstract

Type Vc secretion systems, also known as Trimeric Autotransporter Adhesins (TAAs) are important virulence factors of Gram-negative bacteria.This subclass of bacterial autotransporters forms obligate homotrimers on the surface of bacterial cells, anchored in the outer membrane by the translocator domain at the C-terminus of the protein, through which the rest of the polypeptide is threaded during their biogenesis. During our investigation of the Salmonella adhesin SadA we have discovered that it forms an operon with a small predicted lipoprotein, which we named SadB. As autotransporters are not typically associated with lipoproteins, I was interested in the function of SadB and its interactions with SadA. In this work, I confirmed the prediction that SadB is indeed a lipoprotein, attached to the periplasmic side of the inner membrane. I was able to demonstrate that co-expression of SadB with SadA leads to increased quantity of SadA on the bacterial cell surface, as well as improved folding of the adhesin, which is evidenced by increased protease resistance, as compared to the expression of SadA alone. Additionally, I was able to purify a soluble variant of the protein and obtain a crystal structure with a resolution of 2.45Å. The crystal structure shows, that SadB forms a homotrimer, just as SadA.