Cholesterol-dependent localization of NAP-22 on a neuronal membrane microdomain (Raft)

A membrane microdomain called raft has been under extensive study since the assembly of various signal-transducing molecules into this region has been envisaged. This domain is isolated as a low buoyant membrane fraction afte r the extraction with a nonionic detergent such as Triton X-100. The charac teristic low density of this fraction is ascribed to the enrichment of seve ral lipids including cholesterol. To clear the molecular mechanism of raft formation, several extraction methods were applied to solubilize raft compo nents. Cholesterol extraction using methyl-beta-cyclodextrin was found to b e effective to solubilize NAP-22, a neuron-enriched Ca2+-dependent calmodul in-binding protein as well as one of the main protein components of brain r aft. Purified NAP-22 bound to the liposomes that were made from phosphatidy lcholine and cholesterol. This binding was dependent on the amount of chole sterol in liposomes. Calmodulin inhibited this binding in a dose-dependent manner. These results suggest that the presence of a calcium-dependent regu latory mechanism works on the assembly of raft within the neuron.