A membrane protein insoluble in water was isolated by gel chromatography in the presence of 0.1% sodium dodecyl sulfate (SDS) from chromatophores of a photosynthetic bacterium, Rhodospirillum rubrum. This is one of the major membrane proteins of the chromatophore. The protein was found to bind about four grams of SDS per gram, a value which is more than twice the amount generally observed with protein polypeptides derived from watersoluble globular proteins.
The electrophoretic behavior of the complex between the membrane protein and SDS is abnormal due to this high capacity for binding SDS. Estimation of the molecular weight of this protein by SDS-polyacrylamide gel electrophoresis was thus impossible. Such an anomaly in SDS binding is unlikely to be restricted to the particular membrane protein described in this paper. The possibility of such a deviation from standard behavior in the interaction with SDS should be taken into consideration in studies of other membrane proteins, since SDS is often used both in analytical and preparative procedures.