The major proteins of the Escherichia coli outer cell-envelope membrane. Cyanogen bromide fragments of protein I, composition and order.

The cyanogen bromide fragments of protein I, a major protein of the Escherichia coli outer cell envelope membrane, have been isolated and characterized. There appear to be two methionine-serine or methionine-threonine sequences causing incomplete cleavage but complete conversion of methionine to homoserine. Largely due to the existence of these overlapping fragments the order of 5 of the 6 fragments present could be deduced. None of the fragments exhibits any remarkable low degree of polarity, and the tryptic fingerprint of the largest fragment (comprising about 60% of protein I) also does not show any conspicuous large fraction of lipophilic peptides. It is concluded that the domain of protein I that may be buried in the lipid phase of the outer membrane in all likelihood is not very large, and there is, in fact, no definite proof yet that protein I is a membrane protein sensu stricto.