Structural insight into the giant Ca²âº-binding adhesin SiiE: implications for the adhesion of Salmonella enterica to polarized epithelial cells.

SiiE from Salmonella enterica is a giant 5,559-residue-long nonfimbrial adhesin that is secreted by a type 1 secretion system (T1SS) and initiates bacterial adhesion to polarized host cells. Structural insight has been gained into the 53 bacterial Ig-like (BIg) domains of SiiE, which account for 94% of the entire SiiE sequence. The crystal structure of a fragment comprising BIg domains 50 to 52 of SiiE reveals the BIg domain architecture and highlights two types of SiiE-specific Ca²âº-binding sites. Sequence homology considerations suggest that full-length SiiE interacts with more than 100 Ca²âº ions. Molecular dynamics simulations and single-molecule imaging indicate that Ca²âº binding confers SiiE with a rigid 200 nm rod-like habitus that is required to reach out beyond the Salmonella lipopolysaccharide layer and to promote adhesion to host cells. The crystal structure suggests plausible routes for the establishment of the initial contact between Salmonella and host cells.

Crystallization and preliminary crystallographic analysis of an Ig-domain-encompassing fragment of the giant adhesion protein SiiE from Salmonella enterica.

Salmonella infections can be life-threatening. SiiE is a giant adhesion molecule of 5559 amino acids that is encoded in Salmonella pathogenicity island 4 (SPI4) and that promotes the initial contact between the pathogen and polarized epithelial cells in the intestine of the host. Starting from an engineered deletion version of SiiE (mini-SiiE; 97 kDa), limited proteolysis was used to reproducibly generate a 30 kDa fragment that readily crystallized. Mass spectrometry hints that this fragment spans the predicted Ig domains 50-52 of SiiE. Crystals of both native and selenomethionine-labelled protein could be obtained in space group C2 and diffraction data were recorded to a resolution of 1.85 Å.