The double helix coiled coil structure of murein lipoprotein from Escherichia coli

Abstract

A rod-like structure is proposed for the murein lipoprotein of Escherichia coli, built of two parallel unbroken α-helices arranged in a coiled coil of the same type as in the muscle protein tropomyosin. The amino acid sequence has the required regular pattern of hydrophobic amino acids at intervals of three and four residues and the secondary structure predicted from the sequence is 80% helical. A space-filling model confirms that the coiled coil model is stereochemically reasonable, and energy calculations for a series of coils with different radii suggest that the best structure is one with the helix axes 8.25 Å apart. Energyrefined atomic co-ordinates have been calculated which show that the hydrophobic side-chains form a series of close-packed unstrained contacts between the two helices along the entire length of the sequence. On the basis of this study the hexagonal membrane pore model and the segmented helix model proposed by others seem unlikely. The coiled coil has a strongly hydrophilic outer surface, suggesting that the protein has a watery environment within the E. coli cell envelope and is not strictly a membrane protein. Probably only the fatty acid portion of the lipoprotein penetrates into the lipid region of the outer membrane, so that the protein may act as a tie or a spacer between the lipid and the murein wall.