Size and shape of the Escherichia coli lactose permease measured in filamentous arrays

The Escherichia coli lactose permease has been purified on cation exchanger to contain a minimal amount of phospholipids, i.e., 4-5 mol/mol of permease, in the presence of the detergent dodecyl beta-maltoside at its critical micelle concentration. This preparation is active in galactoside binding. When the detergent level is further reduced by dialysis, the lactose permease forms filaments one molecule wide and up to several micrometers long. The filaments tend to associate laterally to form sheets. Analysis of electron micrographs of negatively stained filamentous arrays indicates an average filament spacing of 51 A and a subunit period of 26-30 A along individual filaments. These values most probably correspond to the dimensions of the lactose permease molecule measured parallel to the membrane plane. In many filaments, the subunits show a stain-penetrated cleft. It suggests that the lactose permease molecule comprises two domains, which may be correlated with internal repeats between the N- and C-terminal halves of the polypeptide sequence.