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 co-expression of human mitochondrial Lon. In mammalian cells, the rate of StAR turnover was proportional to the cell content of Lon protease following treatment with Lon siRNA or over-expression 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 -lactone (cLL, IC₅₀ = 3 µM); 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 cLL but not by epoxomicin. Both inhibitors also blocked Lon-mediated cleavage of the model substrate FITC-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.