StAR-related lipid transfer protein 5 is a protein

Protein

Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...

 that in humans is encoded by the STARD5 gene

Gene

A gene is a molecular unit of heredity of a living organism. It is a name given to some stretches of DNA and RNA that code for a type of protein or for an RNA chain that has a function in the organism. Living beings depend on genes, as they specify all proteins and functional RNA chains...

. The protein is a 213 amino acids long, consisting almost entirely of a StAR-related transfer

StAR-related transfer domain

START is a lipid-binding domain in StAR, HD-ZIP and signalling proteins. The archetypical domain is found in StAR , a mitochondrial protein that is synthesized in steroid-producing cells. StAR and initiates steroid production by mediating the delivery of cholesterol to the first enzyme in...

 (START) domain. It is also part of the StarD4 subfamily of START domain proteins, sharing 34% sequence identity with STARD4

STARD4

StAR-related lipid transfer protein 4 is a soluble protein involved in cholesterol transport. It can transfer up to 7 sterol molecules per minute between artificial membranes.-Function:...

.

Function

The protein is most prevalent in the kidney and the liver where it is found in Kupffer cells. STARD5 binds both cholesterol and 25-hydroxycholesterol and appears to function to redistribute cholesterol to the endoplasmic reticulum

Endoplasmic reticulum

The endoplasmic reticulum is an organelle of cells in eukaryotic organisms that forms an interconnected network of tubules, vesicles, and cisternae...

 with which the protein associates and/or the plasma membrane. Increased levels of StarD5 increase free cholesterol in the cell.

Cholesterol homeostasis is regulated, at least in part, by sterol regulatory element (SRE)-binding proteins (e.g., SREBP1) and by liver X receptor

Liver X receptor

The liver X receptor is a member of the nuclear receptor family of transcription factors and is closely related to nuclear receptors such as the PPARs, FXR and RXR. Liver X receptors are important regulators of cholesterol, fatty acid, and glucose homeostasis...

s (e.g., LXRA

Liver X receptor alpha

Liver X receptor alpha is a nuclear receptor protein that in humans is encoded by the NR1H3 gene .- Expression :...

). Upon sterol depletion, LXRs are inactive and SREBPs are cleaved, after which they bind promoter SREs and activate genes involved in cholesterol biosynthesis and uptake. Sterol transport is mediated by vesicles or by soluble protein carriers, such as steroidogenic acute regulatory protein (STAR

Steroidogenic acute regulatory protein

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

). STAR is homologous to a family of proteins containing a 200- to 210-amino acid STAR-related lipid transfer (START) domain, including STARD5.