Studies on a Protein Component of Guinea Pig Erythrocyte Membranes

Recent investigations of the properties of structural proteins of various cell organelles such as cilia (1), microtubules (2--4), and surface membranes (5, 6) indicate that these organelles are formed in part by proteins which have certain properties in common. These properties are also shared by ac t in -a principal structural protein of muscle tissue. As a result of these observations it has been postulated (6) that such proteins represent a class of actin-like proteins which function as structural units of their respective organelles. In a previous study (7) it was shown that tryptic digestion of red cell membranes resulted in the formation of long polymers which closely resemble F-actin when examined by negative staining and electron microscopy. The appearance of such a fibrous structure is shown in Fig. 1. As a result of this observation, a method for the extraction of this fibrous component from native membranes was developed (5) based on the assumption that the components which formed these actin-like structures also showed the solubility properties of actin. When this procedure was applied to the extracts of intact membrane ghosts of guinea pig erythrocytes, approximately 25 % of the total membrane protein was isolated in low ionic strength media. The extract was found to be made up of a single protein species-named spectrin. Under appropriate conditions spectrin polymerized into fibrous structures identical in appearance to those formed by fibrous actin (Fig. 2).