Assembly landscape of the complete B-repeat superdomain from Staphylococcus epidermidis strain 1457

The accumulation-associated protein (Aap) is the primary determinant of Staphylococcus epidermidis device-related infections. The B-repeat superdomain is responsible for intercellular adhesion that leads to the development of biofilms occurring in such infections. It was recently demonstrated that Zn-induced B-repeat assembly leads to formation of functional amyloid fibrils, which offer strength and stability to the biofilm. Rigorous biophysical studies of Aap B-repeats from S. epidermidis strain RP62A revealed Zn-induced assembly into stable, reversible dimers and tetramers, prior to aggregation into amyloid fibrils. Genetic manipulation is not tractable for many S. epidermidis strains, including RP62A; instead, many genetic studies have used strain 1457. Therefore, to better connect findings from biophysical and structural studies of B-repeats to in vivo studies, the B-repeat superdomain from strain 1457 was examined. Differences between the B-repeats from strains RP62A and 1457 include the number of B-repeats, which has been shown to play a critical role in assembly into amyloid fibrils, as well as the distribution of consensus and variant B-repeat subtypes, which differ in assembly competency and thermal stability. Detailed investigation of the Zn-induced assembly of the full B-repeat superdomain from strain 1457 was conducted using analytical ultracentrifugation. Whereas the previous construct from RP62A (Brpt5.5) formed a stable tetramer prior to aggregation, Brpt6.5 from 1457 forms extremely large stable species on the order of ≈28-mers, prior to aggregation into similar amyloid fibrils. Our data suggest that both assembly pathways may proceed through the same mechanism of dimerization and tetramerization, and both conclude with the formation of amyloid-like fibrils. Discussion of assembly behavior of B-repeats from different strains and of different length is provided with considerations of biological implications.