Structural basis for Zn2+-dependent intercellular adhesion in staphylococcal biofilms

Staphylococcal bacteria, including Staphylococcus epidermidis and Staphylococcus aureus, cause chronic biofilm-related infections. The homologous proteins Aap and SasG mediate biofilm formation in S. epidermidis and S. aureus, respectively. The self-association of these proteins in the presence of Zn2+ leads to the formation of extensive adhesive contacts between cells. This study reports the crystal structure of a Zn2+-bound construct from the self-associating region of Aap. Several unusual structural features include elongated beta-sheets that are solvent-exposed on both faces and the lack of a canonical hydrophobic core. Zn2+-dependent dimers are observed in three distinct crystal forms, formed via pleomorphic coordination of Zn2+ in trans across the dimer interface. These structures illustrate how a long, flexible surface protein is able to form tight intercellular adhesion sites under adverse environmental conditions.