Staphylococcal Bap Proteins Build Amyloid Scaffold Biofilm Matrices in Response to Environmental Signals

Abstract

Biofilms are communities of bacteria that grow encased in an extracellular matrix that often contains proteins. The spatial organization and the molecular interactions between matrix scaffold proteins remain in most cases largely unknown. Here, we report that Bap protein of Staphylococcus aureus self-assembles into functional amyloid aggregates to build the biofilm matrix in response to environmental conditions. Specifically, Bap is processed and fragments containing at least the N-terminus of the protein become aggregation-prone and self-assemble into amyloid-like structures under acidic pHs and low concentrations of calcium. The molten globule-like state of Bap fragments is stabilized upon binding of the cation, hindering its self-assembly into amyloid fibers. These findings define a dual function for Bap, first as a sensor and then as a scaffold protein to promote biofilm development under specific environmental conditions. Since the pH-driven multicellular behavior mediated by Bap occurs in coagulase-negative staphylococci and many other bacteria exploit Bap-like proteins to build a biofilm matrix, the mechanism of amyloid-like aggregation described here may be widespread among pathogenic bacteria.

Amyloid Structures as Biofilm Matrix Scaffolds

“…However, recent results indicate that Bap behaves in a completely different manner. After secretion, Bap is proteolytically processed, and the N-terminal region (amino acids 49 to 819) is released (66) (Fig. 1C).…”

“…These findings define a dual function for Bap, first as a sensor and then as a scaffold protein that promotes biofilm development under specific environmental conditions. This mechanism is substantially different from the curli amyloid assembly machinery for several reasons: (i) Bap secretion does not require a dedicated secretion machinery, because it is efficiently exported when an S. aureus Newman bapnegative strain is complemented with the bap gene (67); (ii) Bap does not require a chaperon to prevent toxic premature assembly inside the cytoplasm, since Bap is assembled extracellularly after protein processing; and (iii) Bap expression does not require a tight transcriptional regulation, since amyloid formation depends not only on the presence of enough Bap but also on specific environmental conditions (66).…”

“…Bap is present in coagulase-negative staphylococci, and many other bacteria use Bap-like proteins to build a biofilm matrix, suggesting that the amyloid-like aggregation described for Bap may be widespread among pathogenic bacteria (58,66).…”

“…The compound (Ϫ)-epi-gallocatechine gallate (EGCG), for example, inhibits the fibrillogenesis of ␣-synuclein and amyloid-␤, two amyloid peptides crucially involved in Alzheimer's and Parkinson's diseases, respectively, and is a potent agent for remodeling of mature amyloid fibrils (90,91). Moreover, the efficiency of EGCG in inhibiting Streptococcus mutans and S. aureus biofilms has been shown (54,66). This gives us the clue that compounds able to inhibit human aberrant amyloids could be active also against bacterial functional amyloids and vice versa.…”

Biofilms 2015: Multidisciplinary Approaches Shed Light into Microbial Life on Surfaces

“…When processed and released, Bap beta-amyloid domains self-assemble into amyloid fibrils and induce bacterial aggregation in response to a decrease in pH during stationaryphase growth. This amyloid behavior is inhibited in the presence of calcium, which is thought to induce compaction, limiting access to proteases that modulate its activity (59).…”