Recrystallized S-layer proteins from a probiotic bacterium A model to probe structural and nanomechanical changes of bacterial surface upon temperature or pH changes

Paracrystalline bacterial surface protein layers (S layers) are common constituents of the bacterial cell wall. These layers originate from the assembly of strain-variable single protein species, the S layer proteins (SLPs), into paracrystalline lattices that eventually cover the entire surface of the bacteria [1]. S layers are thought to play a role in maintaining integrity of the bacteria or in mediating cell-host crosstalk through surface recognition [2]. Thus, S layer-carrying bacteria can show interesting probiotic potential [3]. Such functions may depend on the actual structure of both the SLP and their paracristalline assembly when delivered into the host’s intestine, i.e., after food preparation and ingestion. When isolated, the SLPs spontaneously self-assemble into mono or bilayer paracrystalline arrays on a wide range of substrates and can be used as models for the detailed investigation of bacterial S layers. In the probiotic Propionibacterium freudenreichii, SLPs were shown to play a strain-dependent central role in bacterium/host interactions, including immunomodulation [4]. In the present study, SLP A, isolated from P. freudenreichii strain CIRM BIA 118 was recrystallized at 25°C and pH 6.7 in HEPES buffer then submitted to either increasing temperatures up to 45°C or decreasing pH values down to pH 3.0, relevant to foodmaking and digestion. In the initial conditions, SLP A assembled in hexagonal paracrystalline bilayer as evidenced by atomic force microscopy (AFM). Solid-state Nuclear Magnetic Resonance (NMR) indicated that the internal structure of this protein, (in the S layer array or in solution) exhibited many (~70%) disordered regions, also containing significant amount of bound water. When submitted to heating at 35 or 45°C, the structure of the paracrystalline array was maintained but exhibited decreasing elasticity as probed by atomic force spectroscopy (AFS). When submitted to acidification, the structure and elasticity of the paracrystalline array was maintained at pH 5.0 but varied at pH 3.0. The results were interpreted in terms of changes in the exposed disordered regions of the protein when assembled in the S layer. Such changes may determine the nature and extent of bacterium/host interactions.The authors aknowledge Valérie Briard for mass spectrometry identification of SLP A.