Erythrocytes and their isolated membranes display ATP-dependent endocytosis. To localize the enzymes responsible for this phenomenon, the erythrocyte membranes (ghosts) were fractionated under conditions which retained ATPase activity. Fractionation of the ghosts resulted in three fractions: spectrin-actin, the peripheral proteins soluble in high salt, and the smooth membrane containing integral proteins. On the average, 87% of the protein and 88% of the phosphorus of the original ghosts were recovered in these fractions, and all of the kinds of ATP-splitting activities of the membrane were recovered in the smooth membrane. A tiny ATPase activity, detectable by special methodology in spectrinactin, could have been due to contamination with membranous material. Although the purified spectrin-actin did not have a significant ATPase of its own, it stimulated the Ca2+, Mg2+-ATPase of the smooth membrane significantly, suggesting a cooperative interaction between these two fractions. This segregation of the ATPase activities into the smooth membrane, combined with the energy dependence of endocytosis, showed that the smooth membrane must be involved in the energy production for endocytosis. The possibility that the spectrin-actin filaments cooperate with a myosinlike ATPase in the membrane to generate membrane movements is discussed.

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