Cholesterol-binding START domain of mammalian STARD1, -3 and related proteins
This subfamily includes the steroidogenic acute regulatory protein (StAR)-related lipid transfer (START) domains of STARD1 (also known as StAR) and STARD3 (also known as metastatic lymph node 64/MLN64). The START domain family belongs to the SRPBCC (START/RHO_alpha_C/PITP/Bet_v1/CoxG/CalC) domain superfamily of proteins that bind hydrophobic ligands. SRPBCC domains have a deep hydrophobic ligand-binding pocket. This STARD1-like subfamily has a high affinity for cholesterol. STARD1/StAR can reduce macrophage lipid content and inflammatory status. It plays an essential role in steroidogenic tissues: transferring the steroid precursor, cholesterol, from the outer to the inner mitochondrial membrane, across the aqueous space. Mutations in the gene encoding STARD1/StAR can cause lipid congenital adrenal hyperplasia (CAH), an autosomal recessive disorder characterized by a steroid synthesis deficiency and an accumulation of cholesterol in the adrenal glands and the gonads. STARD3 may function in trafficking endosomal cholesterol to a cytosolic acceptor or membrane. In addition to having a cytoplasmic START cholesterol-binding domain, STARD3 also contains an N-terminal MENTAL cholesterol-binding and protein-protein interaction domain. The MENTAL domain contains transmembrane helices and anchors MLN64 to endosome membranes. The gene encoding STARD3 is overexpressed in about 25% of breast cancers.