Bacillus subtilis Matrix Protein TasA is Interfacially Active, but BslA Dominates Interfacial Film Properties

Microbial growth often occurs within multicellular communities called biofilms, where cells are enveloped by a protective extracellular matrix. Bacillus subtilis serves as a model organism for biofilm research and produces two crucial secreted proteins, BslA and TasA, vital for biofilm matrix formation. BslA exhibits surface-active properties, spontaneously self-assembling at hydrophobic/hydrophilic interfaces to form an elastic protein film, which renders B. subtilis biofilm surfaces water-repellent. TasA is traditionally considered a fiber-forming protein with multiple matrix-related functions. In our current study, we investigate whether TasA also possesses interfacial properties and whether it has any impact on BslA's ability to form an interfacial protein film. Our research demonstrates that TasA indeed exhibits interfacial activity, partitioning to hydrophobic/hydrophilic interfaces, stabilizing emulsions, and forming an interfacial protein film. Interestingly, TasA undergoes interface-induced restructuring similar to BslA, showing an increase in β-strand secondary structure. Unlike BslA, TasA rapidly reaches the interface and forms nonelastic films that rapidly relax under pressure. Through mixed protein pendant drop experiments, we assess the influence of TasA on BslA film formation, revealing that TasA and other surface-active molecules can compete for interface space, potentially preventing BslA from forming a stable elastic film. This raises a critical question: how does BslA self-assemble to form the hydrophobic "raincoat" observed in biofilms in the presence of other potentially surface-active species? We propose a model wherein surface-active molecules, including TasA, initially compete with BslA for interface space. However, under lateral compression or pressure, BslA retains its position, expelling other molecules into the bulk. This resilience at the interface may result from structural rearrangements and lateral interactions between BslA subunits. This combined mechanism likely explains BslA's role in forming a stable film integral to B. subtilis biofilm hydrophobicity.