The S-layer from Bacillus stearothermophilus DSM 2358 functions as an adhesion site for a high-molecular-weight amylase

Abstract

The S-layer lattice from Bacillus stearothermophilus DSM 2358 completely covers the cell surface and exhibits oblique symmetry. During growth of B. stearothermophilus DSM 2358 on starch medium, three amylases with molecular weights of 58,000, 98,000, and 184,000 were secreted into the culture fluid, but only the high-molecular-weight enzyme was found to be cell associated. Studies of interactions between cell wall components and amylases revealed no affinity of the high-molecular-weight amylase to isolated peptidoglycan. On the other hand, this enzyme was always found to be associated with S-layer self-assembly products or S-layer fragments released during preparation of spheroplasts by treatment of whole cells with lysozyme. The molar ratio of S-layer subunits to the bound amylase was approximately 8:1, which corresponded to one enzyme molecule per four morphological subunits. Immunoblotting experiments with polyclonal antisera against the high-molecular-weight amylase revealed a strong immunological signal in response to the enzyme but no cross-reaction with the S-layer protein or the smaller amylases. Immunogold labeling of whole cells with anti-amylase antiserum showed that the high-molecular-weight amylase is located on the outer face of the S-layer lattice. Because extraction of the amylase was possible without disintegration of the S-layer lattice into its constituent subunits, it can be excluded that the enzyme is incorporated into the crystal lattice and participates in the self-assembly process. Affinity experiments strongly suggest the presence of a specific recognition mechanism between the amylase molecules and S-layer protein domains either exposed on the outermost surface or inside the pores. In summary, results obtained in this study confirmed that the S-layer protein from B. stearothermophilus DSM 2358 functions as an adhesion site for a high-molecular-weight amylase.