Membrane proteins of Escherichia coli K-12: two-dimensional polyacrylamide gel electrophoresis of inner and outer membranes.

Protein compositions of the inner and outer membranes of Escherichia coli K-12 have been analyzed by two-dimensional gel electrophoresis in which proteins are separated according to apparent isoelectric point (first dimension) and to apparent molecular weight (second dimension). Membrane proteins except for a pair of major outer membrane proteins (proteins Ia and Ib) were found to be solubilized effectively by lysis buffer containing urea, Triton X-100, ampholines and 2-mercaptoethanol. The latter two proteins could be solubilized after precipitation of membrane fraction with trichloroacetic acid; they formed a pair of spots at an acidic region on the electropherogram. Another major protein of the outer membrane, protein II, was also identified. Most of the inner and outer membrane proteins were shown to be focused at a pH range between 4 and 6.5. Specific protein patterns characteristic for both the inner and outer membranes could thous be visualized by the present system. At least 120 and 50 protein species were detected for the inner and outer membranes, respectively.