The Role of Surface Protein SasG in Biofilm Formation by Staphylococcus aureus

The SasG surface protein of Staphylococcus aureus has been shown to promote the formation of biofilm. SasG comprises an N-terminal A domain and repeated B domains. Here we demonstrate that SasG is involved in the accumulation phase of biofilm, a process that requires a physiological concentration of Zn2+. The B domains and not the A domain are required. Purified recombinant B domain protein can form dimers in vitro in a Zn2+-dependent fashion. Furthermore, the protein can bind to cells that have B domains anchored to their surface and block biofilm formation. The full length SasG protein exposed on the cell surface is processed within the B domains to a limited degree resulting in cleaved proteins of various lengths being released into the supernatant. Some of the released molecules associate with the surface-exposed B domains that remain attached to the cell. Studies using inhibitors and mutants failed to identify any protease that could cause the observed cleavage within the B domains. Extensively purified recombinant B domain protein is very labile and we propose that cleavage occurs spontaneously at labile peptide bonds and that this is necessary for biofilm formation.