Studies on the Lactose Transport System in Escherichia coli

I should like to discuss very briefly the current status of studies in our laboratory of the membrane protein component of the lactose transport system of Escherichia coll. Previous publications (I, 2) have presented evidence that this protein, which can be selectively labeled with radioactive N-ethylmaleimide, is the product of the y gene of the lac operon, the long-sought permease protein. The membrane protein, or M protein, is firmly bound to membrane fragments and can be extracted only with the aid of detergents. In our earlier studies, we used a nonionic detergent, Tri ton X-100, to extract the protein and were able to obtain some purification of the protein by chromatography on ion-exchange resin in the presence of Triton. However, further characterization of the protein using Tri ton extracts proved difficult. Work carried out by Dr. T. D. H. Jones in our laboratory has now shown that electrophoresis in polyacrylamide gels in buffers containing sodium dodecyl sulfate is a useful method for separating the labeled M protein from other membrane proteins. Shapiro et al. (3) have shown that the rate of migration of proteins in the presence of sodium dodecyl sulfate is a function of size, rather than of isoelectric point, and have developed a very useful procedure for determining the size of proteins based on their relative mobility. With this method, we have found that the M protein has a molecular weight of about 30,000 in buffers containing sodium dodecyl sulfate. Since many native proteins dissociate into subunits in the presence of this detergent, it is possible that the native M protein has a size which is some multiple of 30,000. Chromatography on Sephadex G-150 in buffers containing sodium dodecyl sulfate has also proved to be an effective method for the separation and char-