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Many eukaryotic cellular and viral proteins have a covalently attached myristoyl group at the amino terminus. One such protein is recoverin, a calcium sensor in retinal rod cells, which controls the lifetime of photoexcited rhodopsin by inhibiting rhodopsin kinase^,,,,,. Recoverin has a relative molecular mass of 23,000 (M_r 23K), and contains an amino-terminal myristoyl group (or related acyl group) and four EF hands. The binding of two Ca²⁺ ions to recoverin leads to its translocation from the cytosol to the disc membrane,. In the Ca²⁺-free state, the myristoyl group is sequestered in a deep hydrophobic box, where it is clamped by multiple residues contributed by three of the EF hands. We have used nuclear magnetic resonance to show that Ca²⁺ induces the unclamping and extrusion of the myristoyl group, enabling it to interact with a lipid bilayer membrane. The transition is also accompanied by a 45-degree rotation …