The amino acid sequence of major outer membrane protein II* (ompA protein) from Escherichia coli K-12 has been determined. The transmembrane polypeptide consists of 325 residues, resulting in a molecular weight of 35,159. The transmembrane part of the protein is located between residues 1 and 177. In Mis part of te rotein a predominantly lipophilic 27-residue segment exists that perhaps spans the membrane in a mostly a-helical conformation, or a 19-residue stretch of this segment might traverse the membrane linearly. Inside the outer membrane a sequence -Ala-Pro-AlaPro-Ala-Pro-Ala-Pro-exists that, analogous to the -Cys-Pro-ProCys-Pro-sequence in the hinge region of immunoglobulin, could assume the conformation of a polyproline helix. Computer analysis did not reveal a clear overall pattern of internal homology in the protein; besides the -Ala-Pro-repeat, only one local area (two adjacent dodeca eptide segments) shows some repetitiveness. The same analysis did not produce evidence for internal homology in the previously determined sequence of outer membrane protein I (porin) nor was any marked resemblance detected between transmembrane proteins I and II*.

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The amino acid sequence of the pore-forming outer membrane protein I (porin) from Escherichia coli B/r has been determined. The polypeptide contains 340 amino acid residues resulting in a molecular weight of 37,205. The trans-membrane polypeptide has no stretches of nonpolar residues, uninterrupted by charged side chains, longer than 11 amino acid residues. Regarding polarity, the chain can be subdivided into three regions: a distinctly hydrophilic region between residues 1 and 82 (51.2% polarity), a fairly nonpolar region between residues 83 and 194 (33.9% polarity), and a more hydrophilic region up to the COOH terminus (48% polarity). These results are interpreted as evidence against a simple transmembrane structure in which the membrane is spanned by a single contiguous sequence of hydrophobic amino acids, as has been proposed, or example, for glycophorin. The cell envelope of Gram-negative bacteria possesses, in addition to the plasma membrane, an outer membrane which has perhaps more correctly also been called a porous skeletal organ (see ref. 1 for recent review) (2). This porosity is provided by proteins, the porins (e.g., refs. 3-5), which form hydrophilic channels allowing the diffusion of various low molecular weight solutes. In Escherichia coli B/r, protein 1 (6) [closely related to Rosenbusch's matrix protein from E. coli BE, (7)] is the porin responsible for the existence of these channels, which have a diameter of about 0.9 nm (8, 9). Aside from the fact that information is sparse regarding structure-function relationships of integral membrane proteins in general, the porins pose a number of interesting questions in addition to those connected with their physiological functions. In E. coli K-12 a whole family of such proteins of similar size and properties exists, and it seems that, under usual laboratory conditions, several of the corresponding structural genes are silent or nearly so (10-13). These genes are not clustered on the E. coli chromosome, and if they have arisen by duplications they might allow some insight into the evolution of this chromosome (11, 13-17). Furthermore, several of these proteins can serve as at least parts of phage receptors (18-21) and they are required for an apparent uptake of protein into the cell: mutants lacking certain such polypeptides are highly tolerant to several colicins (e.g., refs. 22 and 23). It is not known what constitutes a phage receptor area on such proteins and what is their function in colicin sensitivity. Finding answers to all these and related questions would be helped by a knowledge of the amino acid sequence of such a protein. It is also likely that the gene for the protein under study will soon become available by DNA cloning. The determination of the DNA sequence, of much interest because of the unknown control region(s), should of course also be much aided by knowledge of the primary structure of the protein. We have determined the sequence of protein I from E. coli B/r and here present its primary structure. [Strains of E. coli B/r can dif...