Lipoprotein from the outer membrane of Escherichia coli: purification, paracrystallization, and some properties of its free form

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Abstract
In the envelope of E. coli, is a lipoprotein of MW 7200 as a major envelope protein. This lipoprotein was previously shown to exist in 2 different forms in the outer membrane of E. coli: the free form and the bound form, which is covalently linked to the peptidoglycan. The free form of the lipoprotein was purified and paracrystallized by adding acetone to a sodium dodecyl sulfate solution in the presence of Mg2+. The paracrystals were needle shaped. An electron micrograph of the negatively stained paracrystals showed a highly ordered ultrastructure. The chemical structure of the free form was compared with that of the bound form by the amino acid composition, the fatty acid composition and the peptide analysis after cyanogen bromide cleavage. The .alpha.-helical content of the free form of the lipoprotein was measured from the circular dichroism spectrum of the lipoprotein in 0.01% sodium dodecyl sulfate and found to be 87%. Using the purified lipoprotein as antigen, antiserum against the free form of the lipoprotein was obtained. Immunoprecipitation of the lipoprotein with the antiserum was very specific, since only the free form of the lipoprotein was found as a major peak when the antiserum was reacted with the whole envelope proteins solubilized in 0.2% sodium dodecyl sulfate and the immunoprecipitate thus formed was analyzed by polyacryamide gel electrophoresis.