The life cycle of the steroidogenic acute regulatory (StAR) protein: from transcription through proteolysis.
The Steroidogenic Acute Regulatory (StAR) protein is a mitochondrial protein required for the transport of cholesterol substrate to the P450scc enzyme located in the inner mitochondrial membranes of steroid producing cells. This study suggests that the acute regulation of the rodent StAR gene in the ovary is mediated by two factors, C/EBPbeta and GATA-4. Once translated, the StAR precursor protein is either imported into the mitochondria, or it is rapidly degraded in the cytosol. We predicted that in order to perpetuate StAR activity cycles, imported StAR should turn over rapidly to avoid a potentially harmful accumulation of the protein in sub-mitochondrial compartments. Pulse-chase experiments in metabolically labeled cells showed that: (a) the turnover rate of mature mitochondrial StAR protein (30 kDa) is much faster (t(1/2) = 4-5 h) than that of other mitochondrial proteins; (b) dissipation of the inner membrane potential (-delta psi) by carbonyl cyanide m-chlorophenylhydrazone (mCCCP) accelerates the mitochondrial degradation of StAR; (c) unexpectedly, the mitochondrial degradation of StAR is inhibited by MG132 and lactacystin, but not by epoxomicin. Furthermore, StAR degradation becomes inhibitor-resistant two hours after import. Therefore, these studies suggest a bi-phasic route of StAR turnover in the mitochondria. Shortly after import, StAR is degraded by inhibitor-sensitive protease(s) (phase I), whereas at later times, StAR turnover proceeds to completion through an MG132-resistant proteolytic activity (phase II). Collectively, this study defines StAR as a unique protein that can authentically be used to probe multiple proteolytic activities in mammalian mitochondria.
Granot
Zvi
Z
Department of Biological Chemistry, The Alexander Silberman Institute of Life Sciences, The Hebrew University of Jerusalem, Jerusalem 91904, Israel.
Silverman
Eran
E
Friedlander
Ruth
R
Melamed-Book
Naomi
N
Eimerl
Sarah
S
Timberg
Rina
R
Hales
Karen H
KH
Hales
Dale B
DB
Stocco
Douglas M
DM
Orly
Joseph
J
D000818:Animals; D001692:Biological Transport; D019556:COS Cells; D002258:Carbonyl Cyanide m-Chlorophenyl Hydrazone / Q000494:pharmacology; D002460:Cell Line; D015853:Cysteine Proteinase Inhibitors / Q000494:pharmacology; D005260:Female; D006107:Granulosa Cells / Q000378:metabolism; D006801:Humans; D007976:Leupeptins / Q000494:pharmacology; D008564:Membrane Potentials / Q000187:drug effects; D051379:Mice; D008928:Mitochondria / Q000378:metabolism / Q000502:physiology; D010447:Peptide Hydrolases / Q000378:metabolism*; D010750:Phosphoproteins / Q000235:genetics* / Q000378:metabolism*; D014158:Transcription, Genetic / Q000502:physiology; D000098682:Steroidogenic Acute Regulatory Protein
D015853:Cysteine Proteinase Inhibitors; D007976:Leupeptins; D010750:Phosphoproteins; D000098682:Steroidogenic Acute Regulatory Protein; D002258:Carbonyl Cyanide m-Chlorophenyl Hydrazone; D010447:Peptide Hydrolases; C072553:benzyloxycarbonylleucyl-leucyl-leucine aldehyde