An update on the mechanism of action of the Steroidogenic Acute Regulatory (StAR) protein.

The Steroidogenic Acute Regulatory (StAR) protein has been demonstrated to be an indispensable component in the acute regulation of steroid hormone biosynthesis. The StAR protein, which is rapidly synthesized in response to tropic hormone stimulation, apparently functions by mediating the transfer of the substrate for all steroid hormones, cholesterol, from the outer to the inner mitochondrial membrane where it is cleaved to pregnenolone, the first steroid formed. This transfer is the regulated and rate limiting step in steroidogenesis. Mutations in the StAR gene have been shown to be the only cause of the potentially fatal disease, lipoid congenital adrenal hyperplasia, in which the affected individual can synthesize virtually no steroids. One of the most interesting and important areas of studies on this protein encompasses the mechanism whereby the StAR protein mediates the transfer of cholesterol to the inner mitochondrial membrane. While this mechanism remains unknown at this time, this review will attempt to bring this aspect of StAR function up to date.