Bacterial S-layer protein coupling to lipids: X-ray reflectivity and grazing incidence diffraction studies

The coupling of bacterial surface (S)-layer proteins to lipid membranes is studied in molecular detail for proteins from Bacillus sphaericus CCM2177 a nd B. coagulans E38-66 recrystallized at dipalmitoylphosphatidylethanolamin e (DPPE) monolayers on aqueous buffer. A comparison of the monolayer struct ure before and after protein recrystallization shows minimal reorganization of the lipid chains. By contrast, the lipid headgroups show major rearrang ements. For the B. sphaericus CCM2177 protein underneath DPPE monolayers, x -ray reflectivity data suggest that amino acid side chains intercalate the lipid headgroups at least to the phosphate moieties, and probably further b eyond. The number of electrons in the headgroup region increases by more th an four per lipid. Analysis of the changes of the deduced electron density profiles in terms of a molecular interpretation shows that the phosphatidyl ethanolamine headgroups must reorient toward the surface normal to accommod ate such changes. In terms of the protein structure (which is as yet unknow n in three dimensions), the electron density profile reveals a thickness I- z approximate to 90 Angstrom of the recrystallized S-layer and shows water- filled cavities near its center. The protein volume fraction reaches maxima of >60% in two horizontal sections of the S-layer, close to the lipid mono layer and close to the free subphase. In between it drops to similar to 20% . Four S-layer protein monomers are located within the unit cell of a squar e lattice with a spacing of similar to 131 Angstrom.