title: Turnover of StAR protein: roles for the proteasome and mitochondrial proteases.
abstract: Steroidogenic acute regulatory protein (StAR) is a mitochondrial protein essential for massive synthesis of steroid hormones in the adrenal and the gonads. Our studies suggest that once synthesized on free polyribosomes, StAR preprotein either associates with the outer mitochondrial membrane to mediate transfer of cholesterol substrate required for steroidgenesis, or it is degraded by the proteasome. Proteasome inhibitors can prevent the turnover of StAR preprotein and other matrix-targeted preproteins. Once imported, excessive accumulation of inactive StAR in the matrix is avoided by a rapid turnover. Unexpectedly, mitochondrial StAR turnover can be inhibited by two proteasome inhibitors, i.e., MG132 and clasto-lactacystin beta-lactone, but not epoxomicin. Use of those inhibitors and immuno-electron microscopy data enabled a clear distinction between two pools of intra-mitochondrial StAR, one degraded by matrix protease(s) shortly after import, while the rest of the protein undergoes a slower and inhibitor resistant degradation following translocation onto to the matrix face of the inner membranes.
authors.affiliation: Department of Biological Chemistry, The Alexander Silberman Institute of Life Sciences, The Hebrew University of Jerusalem, Jerusalem 91904, Israel.
authors.forename: Zvi
authors.initials: Z
authors.lastname: Granot
authors.forename: Naomi
authors.initials: N
authors.lastname: Melamed-Book
authors.forename: Assaf
authors.initials: A
authors.lastname: Bahat
authors.forename: Joseph
authors.initials: J
authors.lastname: Orly
mesh_terms: D000818:Animals; D006801:Humans; D007783:Lactones / Q000494:pharmacology; D007976:Leupeptins / Q000494:pharmacology; D008928:Mitochondria / Q000201:enzymology* / Q000378:metabolism; D051336:Mitochondrial Membranes / Q000201:enzymology / Q000378:metabolism; D009842:Oligopeptides / Q000494:pharmacology; D010447:Peptide Hydrolases / Q000378:metabolism*; D010750:Phosphoproteins / Q000378:metabolism*; D011480:Protease Inhibitors / Q000494:pharmacology; D046988:Proteasome Endopeptidase Complex / Q000378:metabolism*; D000098682:Steroidogenic Acute Regulatory Protein
chemical_list: D007783:Lactones; D007976:Leupeptins; D009842:Oligopeptides; D010750:Phosphoproteins; D011480:Protease Inhibitors; D000098682:Steroidogenic Acute Regulatory Protein; C098869:clasto-lactacystin beta-lactone; D010447:Peptide Hydrolases; D046988:Proteasome Endopeptidase Complex; C072553:benzyloxycarbonylleucyl-leucyl-leucine aldehyde; C078846:epoxomicin