Tracking the role of a star in the sky of the new millennium.

The steroidogenic acute regulatory protein is indispensable for the biosynthesis of steroid hormones. Steroidogenic acute regulatory protein mediates the rate-limiting step in steroidogenesis, the transfer of cholesterol from the outer mitochondrial membrane to the inner mitochondrial membrane where it is cleaved to pregnenolone. Its essential role in steroidogenesis was shown when it was discovered that mutations in the steroidogenic acute regulatory protein gene in humans cause the lipoid form of congenital adrenal hyperplasia, a potentially lethal disease resulting from an inability to synthesize steroids. Also, the steroidogenic acute regulatory protein null mouse has a phenotype that is essentially the same as that observed with human mutations. Studies on the regulation of the expression of the steroidogenic acute regulatory protein gene has enjoyed considerable progress, yet the complexity of this regulation indicates that much work remains. The mechanism whereby steroidogenic acute regulatory protein mediates the transfer of cholesterol to the inner mitochondrial membrane remains a mystery, but the recent solving of the structure of the cholesterol transferring domain of a steroidogenic acute regulatory protein homolog coupled with structure-function studies of steroidogenic acute regulatory protein in natural and synthetic membranes has allowed for at least two models to be proposed. This review will briefly attempt to summarize what is currently known about the regulation of the steroidogenic acute regulatory protein gene and its mechanism of action, fully understanding that in both areas considerable gaps in our knowledge remain.