title: Steroidogenic acute regulatory protein has a more open conformation than the independently folded smaller subdomains.
abstract: The acute steroidogenic response, which produces steroids in response to stress, requires the steroidogenic acute regulatory protein (StAR). StAR, a mitochondrial matrix protein, acts on the outer mitochondrial membrane (OMM) to facilitate the movement of cholesterol from the outer to inner mitochondrial membrane via an unknown mechanism. The N-terminal sequence was reported to be nonessential for activity. We show that alteration of the StAR amino-terminal sequence does not change the thermodynamic stability of StAR but offers protection from proteolytic degradation. A longer association between StAR and the OMM strengthens the interaction with cholesterol. Far-UV CD spectra showed that the smaller fragments of StAR domains were less alpha-helical compared to N-62 StAR but were structured as determined by limited proteolysis followed by mass spectrometry. The START domain consisting of amino acids 63-193 also exhibited protease protection for amino acids 84-193. The Stern-Volmer quenching constant (K(SV)) of the N-62 StAR protein is 12.1 x 10(5) M(-1), with all other START fragments having significantly smaller K(SV) values ranging from 6 to 10 x 10(5) M(-1), showing that N-62 StAR has a more open conformation. Only N-62 StAR protein is stabilized with cholesterol having an increased DeltaH value of -5.6 +/- 0.3 kcal/mol at 37 degrees C. These findings demonstrate a mechanism in which StAR is stabilized at the OMM by cholesterol to initiate its massive import into mitochondria.
authors.affiliation: Department of Biomedical Sciences, Mercer University School of Medicine and Memorial University Medical Center, Savannah, Georgia 31404, USA. bose_hs@mercer.edu
authors.forename: Himangshu S
authors.initials: HS
authors.lastname: Bose
authors.forename: Randy M
authors.initials: RM
authors.lastname: Whittal
authors.forename: Dilip
authors.initials: D
authors.lastname: Debnath
authors.forename: Mahuya
authors.initials: M
authors.lastname: Bose
mesh_terms: D001693:Biological Transport, Active / Q000502:physiology; D002784:Cholesterol / Q000737:chemistry / Q000378:metabolism; D003593:Cytoplasm / Q000737:chemistry / Q000378:metabolism; D033681:Mitochondrial Membrane Transport Proteins / Q000737:chemistry* / Q000378:metabolism / Q000502:physiology; D010750:Phosphoproteins / Q000737:chemistry* / Q000378:metabolism / Q000502:physiology; D011487:Protein Conformation; D017510:Protein Folding; D017434:Protein Structure, Tertiary / Q000502:physiology; D021381:Protein Transport / Q000502:physiology; D000098682:Steroidogenic Acute Regulatory Protein
chemical_list: D033681:Mitochondrial Membrane Transport Proteins; D010750:Phosphoproteins; D000098682:Steroidogenic Acute Regulatory Protein; D002784:Cholesterol