Caveolin-1 is a palmitoylated protein involved in assembly of signaling molecules in plasma membrane subdomains termed caveolae and in intracellular cholesterol transport. Three cysteine residues in the C terminus of caveolin-1 are subject to palmitoylation, which is not necessary for caveolar targeting of caveolin-1. Protein palmitoylation is a post-translational and reversible modification that may be regulated and that in turn may regulate conformation, membrane association, protein-protein interactions, and intracellular localization of the target protein. We have undertaken a detailed analysis of [ 3 H]palmitate incorporation into caveolin-1 in aortic endothelial cells. The linkage of palmitate to caveolin-1 was hydroxylamine-sensitive and thus presumably a thioester bond. However, contrary to expectations, palmitate incorporation was blocked completely by the protein synthesis inhibitors cycloheximide and puromycin. In parallel experiments to show specificity, palmitoylation of aortic endothelial cell-specific nitric-oxide synthase was unaffected by these reagents. Inhibitors of protein trafficking, brefeldin A and monensin, blocked caveolin-1 palmitoylation, indicating that the modification was not cotranslational but rather required caveolin-1 transport from the endoplasmic reticulum and Golgi to the plasma membrane. In addition, immunophilin chaperones that form complexes with caveolin-1, i.e. FK506-binding protein 52, cyclophilin A, and cyclophilin 40, were not necessary for caveolin-1 palmitoylation because agents that bind immunophilins did not inhibit palmitoylation. Pulse-chase experiments showed that caveolin-1 palmitoylation is essentially irreversible because the release of [ Originally named to describe membrane invaginations at the cell surface, caveolae are specialized plasmalemmal domains rich in glycosphingolipids, cholesterol, and lipid-anchored membrane proteins. In endothelial cells (EC) 1 , caveolae have a striated coat, the major protein of which is caveolin-1 (1). Caveolin-1 is involved in cholesterol trafficking (2), oligomerizes to form a scaffold for assembly of signaling molecules including receptors, signal transducers, and effectors (3), and regulates the activation state of these signaling complexes (4).Lipid modification, including palmitoylation, is an important mechanism targeting signaling proteins to caveolae. Among the palmitoylated proteins in caveolae are heterotrimeric Gprotein ␣-subunits (e.g. ␣ s , ␣ i , ␣ o , and ␣ q ), G-protein linked receptors, regulator of G-protein signaling proteins, p21 ras , nonreceptor tyrosine kinases, and EC-specific nitric-oxide synthase (eNOS) (5). Palmitoylation of several of these proteins has been shown to be a post-translational and reversible modification that is also subject to regulation (6). The post-translational nature of palmitoylation has been shown by cell culture experiments in which palmitate labeling occurs even in the presence of protein synthesis inhibitors (7). These experiments clearly distinguish protein palmitoyl...