Steroidogenic acute regulatory protein

The steroidogenic acute regulatory protein, commonly referred to as StAR (STARD1), is a transport protein that regulates cholesterol transfer within the mitochondria, which is the rate-limiting step in the production of steroid hormones.

Function
Cholesterol needs to be transferred from the outer mitochondrial membrane to the inner membrane where cytochrome P450scc enzyme is located to split off the cholesterol side chain, which is the first enzymatic step in all steroid synthesis. The aqueous phase between these two membranes cannot be crossed by the lipophilic cholesterol, unless certain proteins assist in this process. A number of proteins have historically been proposed to facilitate this transfer including: sterol carrier protein 2 (SCP2), steroidogenic activator polypeptide (SAP),  peripheral benzodiazepine receptor (PBR), and StAR. It is now clear that this process is primarily mediated by StAR.

Structure
In humans, the gene for StAR is located on chromosome 8p11.2 and the protein has 285 amino acids. The signal sequence of StAR that targets it to the mitochondria is clipped off in two steps with import into the mitochondria. Phosphorylation at the serine at position 195 increases its activity.

The domain of StAR important for promoting cholesterol transfer is the StAR-related transfer domain (START domain). StAR is the prototypic member of the START domain family of proteins. It is hypothesized that the START domain forms a pocket in StAR that binds single cholesterol molecules for delivery to P450scc.

The closest homolog to StAR is MLN64.

Production
StAR is a mitochondrial protein that is rapidly synthesized in response to stimulation of the cell to produce steroid. Hormones that stimulate its production depend on the cell type and include luteinizing hormone (LH), ACTH and angiotensin II.

StAR has thus far been found in all tissues that can produce steroids, including the adrenal cortex, the gonads, the brain and placenta. One known exception is the human placenta.

Alcohol suppresses StAR activity.

At the cellular level, StAR is synthesized typically in response to activation of the cAMP second messenger system, although other systems can be involved even independently of cAMP.

Pathology
Mutations in the gene for StAR cause lipoid congenital adrenal hyperplasia, in which patients produce little steroid and can die shortly after birth. All known mutations disrupt StAR function by altering its START domain.

At the cellular level, the lack of StAR results in a pathologic accumulation of lipid within cells, especially noticeable in the adrenal cortex as seen in the mouse model. The testes is modestly affected. Early in life, the ovary is spared as it does not express StAR until puberty. After puberty, lipid accumulations and hallmarks of ovarian failure are noted.

StAR-Independent Steroidogenesis
While loss of functional StAR in the human and the mouse catastrophically reduces steroid production, it does not eliminate all of it, indicating the existence of StAR-independent pathways for steroid generation. Aside from the human placenta, these pathways are considered minor for endocrine production.

It is unclear what factors catalyze StAR-independent steroidogenesis. Candidates include oxysterols which can be freely converted to steroid and the ubiquitous MLN64.