Perspectives and limitations of resolutions-reconstitution experiments.

Reconstitutions of membranous activities can tell us how many components are required and what their functions are. The mitochondrial proton pump is used as an example. Moreover, the biological activity, such as Pi transport, can be used in reconstituted vesicles as an assay during the isolation of the transporter. Reconstitution experiments reveal the importance of membrane asymmetry and allow us to study conditions of vectorial assembly. The mechanism of action of ion pumps has been successfully analyzed in reconstituted liposomes. We can study the movement of ions and the electrogenicity of the system without interference by other unrelated processes. Based on studies with the resolved Ca2+-ATPase of sarcoplasmic reticulum, we propose a novel formulation of the mechanism of ATP-driven ion pumps in which cyclic binding of Mg2+ plays a key role.