Conformational studies on murein-lipoprotein from the outer membrane of Escherichia coli.

Conformational studies on an isolated integral membrane protein are reported. Lipoprotein of Escherichia coli outer membrane was released from murein by treatment with either lysozyme or trypsin. The isolated lysozyme-released lipoprotein (lipoprotein I) contained 2 or 3 muropeptides covalently linked at the C-terminal end, while the trypsin-released lipoprotein (lipoprotein II) was free of muropeptides and lacked the C-terminal peptide Tyr-Arg-Lys. Circular dichroism spectra of the two preparations were essentially identical, and they show an alpha-helix content of about 80%. According to calculations based on the Chou-Fasman rules for proteins of known sequence, lipoprotein is 64% alpha-helix and 15% beta-structure. Infrared spectroscopy qualitatively supports these values. The conformation was stable in the pH range of 5 - 12. Danaturation of lipoprotein by heat, 8 M urea, or sodium dodecylsulphate was a fully reversible, cooperative process. The thermal denaturation of lipoprotein occurs in two steps with transition points at 79.4 degrees C for lipoprotein I and at 85.1 degrees C for lipoprotein II. Lioprotein markedly changes conformation at dodecylsulphate concentrations where micelle formation sets in. The unusual behaviour of the lipoprotein convormation in sodium dodecylsulphate is discussed in relation to the lipoprotein conformation and aggregation within the membrane.