S-layer protein. The S-layer is a paracrystalline mono-layered assembly of proteins which coats the surface of bacteria. This S-layer, also called A-layer, confers virulence to the bacterium infecting salmonid and non-salmonid fish. It binds specifically to laminin and fibronectin. Monomer. Forms oligomers at high protein or low salt concentrations. This bacterium is covered by a S-layer with tetragonal symmetry. Tethered to the outer membrane by interaction with O-side chain of the LPS. This S-layer protein is composed of two distinct structural domains: a larger mass trypsin-resistant N-terminal, containing the binding region for extracellular matrix proteins, and a smaller C-terminal domain with intermediate resistance to trypsin.