Lipid-binding START domain of mammalian STARD4, -5, -6, and related proteins
This subfamily includes the steroidogenic acute regulatory protein (StAR)-related lipid transfer (START) domains of mammalian STARD4, -5, and -6. 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. STARD4 plays an important role in steroidogenesis, trafficking cholesterol into mitochondria. It specifically binds cholesterol, and demonstrates limited binding to another sterol, 7a-hydroxycholesterol. STARD4 and STARD5 are ubiquitously expressed, with highest levels in liver and kidney. STRAD5 functions in the kidney within the proximal tubule cells where it is associated with the Endoplasmic Reticulum (ER), and may participate in ER-associated cholesterol transport. It binds cholesterol and 25-hydroxycholesterol. Expression of the gene encoding STARD5 is increased by ER stress, and its mRNA and protein levels are elevated in a type I diabetic mouse model of human diabetic nephropathy. STARD6 is expressed in male germ cells of normal rats, and in the steroidogenic Leydig cells of perinatal hypothyroid testes. It may play a pivotal role in the steroidogenesis as well as in the spermatogenesis of normal rats. STARD6 has also been detected in the rat nervous system, and may participate in neurosteroid synthesis.