TY - JOUR
T1 - Turnover of mitochondrial Steroidogenic Acute Regulatory (StAR) protein by lon protease
T2 - The unexpected effect of proteasome inhibitors
AU - Granot, Zvi
AU - Kobiler, Oren
AU - Melamed-Book, Naomi
AU - Eimerl, Sarah
AU - Bahat, Assaf
AU - Lu, Bin
AU - Braun, Sergei
AU - Maurizi, Michael R.
AU - Suzuki, Carolyn K.
AU - Oppenheim, Amos B.
AU - Orly, Joseph
PY - 2007/9
Y1 - 2007/9
N2 - Steroidogenic acute regulatory protein (StAR) is a vital mitochondrial protein promoting transfer of cholesterol into steroid making mitochondria in specialized cells of the adrenal cortex and gonads. Our previous work has demonstrated that StAR is rapidly degraded upon import into the mitochondrial matrix. To identify the protease(s) responsible for this rapid turnover, murine StAR was expressed in wild-type Escherichia coli or in mutant strains lacking one of the four ATP-dependent proteolytic systems, three of which are conserved in mammalian mitochondria - ClpP, FtsH, and Lon. StAR was rapidly degraded in wild-type bacteria and stabilized only in lon -mutants; in such cells, StAR turnover was fully restored upon coexpression of human mitochondrial Lon. In mammalian cells, the rate of StAR turnover was proportional to the cell content of Lon protease after expression of a Lon-targeted small interfering RNA, or overexpression of the protein. In vitro assays using purified proteins showed that Lon-mediated degradation of StAR was ATP-dependent and blocked by the proteasome inhibitors MG132 (IC50 = 20 μM) and clasto-lactacystin β-lactone (cLβL, IC50 = 3 μM); by contrast, epoxomicin, representing a different class of proteasome inhibitors, had no effect. Such inhibition is consistent with results in cultured rat ovarian granulosa cells demonstrating that degradation of StAR in the mitochondrial matrix is blocked by MG132 and cLβL but not by epoxomicin. Both inhibitors also blocked Lon-mediated cleavage of the model substrate fluorescein isothiocyanate-casein. Taken together, our former studies and the present results suggest that Lon is the primary ATP-dependent protease responsible for StAR turnover in mitochondria of steroidogenic cells.
AB - Steroidogenic acute regulatory protein (StAR) is a vital mitochondrial protein promoting transfer of cholesterol into steroid making mitochondria in specialized cells of the adrenal cortex and gonads. Our previous work has demonstrated that StAR is rapidly degraded upon import into the mitochondrial matrix. To identify the protease(s) responsible for this rapid turnover, murine StAR was expressed in wild-type Escherichia coli or in mutant strains lacking one of the four ATP-dependent proteolytic systems, three of which are conserved in mammalian mitochondria - ClpP, FtsH, and Lon. StAR was rapidly degraded in wild-type bacteria and stabilized only in lon -mutants; in such cells, StAR turnover was fully restored upon coexpression of human mitochondrial Lon. In mammalian cells, the rate of StAR turnover was proportional to the cell content of Lon protease after expression of a Lon-targeted small interfering RNA, or overexpression of the protein. In vitro assays using purified proteins showed that Lon-mediated degradation of StAR was ATP-dependent and blocked by the proteasome inhibitors MG132 (IC50 = 20 μM) and clasto-lactacystin β-lactone (cLβL, IC50 = 3 μM); by contrast, epoxomicin, representing a different class of proteasome inhibitors, had no effect. Such inhibition is consistent with results in cultured rat ovarian granulosa cells demonstrating that degradation of StAR in the mitochondrial matrix is blocked by MG132 and cLβL but not by epoxomicin. Both inhibitors also blocked Lon-mediated cleavage of the model substrate fluorescein isothiocyanate-casein. Taken together, our former studies and the present results suggest that Lon is the primary ATP-dependent protease responsible for StAR turnover in mitochondria of steroidogenic cells.
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U2 - 10.1210/me.2005-0458
DO - 10.1210/me.2005-0458
M3 - Article
C2 - 17579211
AN - SCOPUS:34548446023
SN - 0888-8809
VL - 21
SP - 2164
EP - 2177
JO - Molecular Endocrinology
JF - Molecular Endocrinology
IS - 9
ER -