Functional analysis of the Tsh autotransporter from an avian pathogenic Escherichia coli strain

The temperature-sensitive hemagglutinin (Tsh) is an autotransporter protein secreted by avian-pathogenic Escherichia coli strains that colonize the respiratory tract and lead to airsacculitis, pericarditis, and colisepticemia. It is synthesized as a 140-kDa precursor protein, whose processing results in a 106-kDa passenger domain (Tsh(s)) and a 33-kDa beta-domain (Tsh(beta)). The presence of a conserved 7-amino-acid serine protease motif within Tsh(s) classifies the protein in a subfamily of autotransporters, known as serine protease autotransporters of the Enterobacteriaceae. In this study, we report that purified Tsh(s) is capable of adhering to red blood cells, hemoglobin, and the extracellular matrix proteins fibronectin and collagen IV. We also demonstrate that Tsh(s) exerts proteolytic activity against casein, and we provide experimental evidence demonstrating that serine 259 is essential for the protease function. However, this residue is not required for adherence to substrates, and its replacement by an alanine does not abolish binding activity. In summary, our results demonstrate that Tsh is a bifunctional protein with both adhesive and proteolytic properties.