The outer membranes of Escherichia coli and Salmonella typhimurium contain several proteins that have been shown to produce rather large channels for diffusion [ 11 .
One class, called "porins," has apparent molecular weights in the range of 34-37,000, and, when reconstituted into phospholipid-lipopolysaccharide bilayers, produced channels allowing the diffusion of sugars, sugar phosphates, nucleotides, amino acids, and polyethyleneglycols as long as their molecular weights were less than 600 . These results suggested, at least to us, that porin channels were largely nonspecific and acted as simple molecular sieves. Outer membrane proteins of another class, in contrast, appear to be involved in the transmembrane diffusion of special classes of solutes [ 1 , 4 ] . At least one member of this group, the phage h receptor protein (or lamB protein) of an apparent molecular weight of 50,000, was shown to produce transmembrane channels in reconstituted planar bilayers and vesicles [ 5 , 6 ] , and to function in the transport of oligosaccharides of maltose series in intact cells . It is tempting to assume, therefore, that these proteins form specific channels quite different from the nonspecific, porin channels. However, reports from several laboratories have presented conclusions contrary to the hypotheses presented above, and suggested that porin channels had some solute specificity whereas the h receptor channel was nonspecific [6-81 . The purpose of this short review is to critically analyze the results from other laboratories, and to summarize our recent data pertaining to the question of the specificity of protein channels in the outer membrane. channel is a physicochemical process, it will obviously be influenced by the gross physicochemical properties of the solute, such as its size, hydrophobicity, and charge. Thus, if two solutes differing in some of these properties are tested, their diffusion rates are likely to be different. This does not indicate the presence of specificity. We use the word specificity in the sense traditionally used in biochemistry, ie, to mean configurational specificity or stereospecificity. Thus, a specific channel should discriminate between solutes that are similar in their gross physicochemical properties.
We should first define the word "specificity." Since diffusion through water-filled