Submission Number: JOS-4-11-4

Abstract Number: 434

FATE AND TURNOVER OF STEROIDOGENIC ACUTE REGULATORY (STAR) PROTEIN: UNIQUE DEGRADATION BY THE MITOCHONDRIAL QUALITY CONTROL MACHINERY.

J Orly*, Z Granot, R Friedlander, N Melamed-Book and S Eimerl

Department of Biological Chemistry, Institute of Life Sciences, The Hebrew University of Jerusalem, Jerusalem, Israel ¹

Abstract:
StAR facilitates cholesterol transfer into the inner membranes of steroidogenic mitochondria, where it serves as substrate for the first and rate limiting reaction catalyzing steroid biosynthesis. Current models assume that cholesterol transfer is mediated by newly translated StAR preprotein prior to its import and processing in the mitochondrion. According to this perception, StAR import aims to "switch off" StAR bioactivity by removing it from the mitochondrial surface. We hypothesized that in order to perpetuate StAR activity cycles, imported StAR is expected to be rapidly cleared to avoid excessive and potentially harmful accumulation of the "useless" protein in the sub-mitochondrial compartments. To test this hypothesis we studied turnover rates of ³⁵S-labeled StAR in FSH treated ovarian granulosa cells: (a) StAR preprotein (37 kDa) remains shortly in the cytosol (t1/2= 3 min) before it is imported into the mitochondrion. (b) Degradation of StAR preprotein in the cytosol is inhibited by MG132 and epoxomicin, suggesting the involvement of the ubiquitin/proteasome proteolytic pathway. (c) Once imported, the turnover rate of mature StAR (30 kDa) inside the mitochondrion (t1/2= 3 h) is dramatically shorter than that of P450scc, cytochrome c, or recombinant matrix targeted GFP; (d) Dissipation of Dy by oxphos uncoupler, mCCCP, accelerates the degradation of StAR (t1/2= 1.5 h), but has no effect on other proteins; (e) Unexpectedly, the degradation of mitochondrial StAR is inhibited by selective inhibitors of the 26S proteasome, MG132 and lactacystin. Together with corroborating confocal microscopy studies, these findings propose that following import, StAR degradation is launched by probably several mitochondrial proteases, some of which can be blocked by proteasomal inhibitors. Also, our biochemical and electron microscope evidence suggest that following import into the matrix, StAR tends to associate with the inner mitochondrial membranes and affect their selective properties. Supported by The Israel Science Foundation (#547/97) .

Keywords: StAR, steroidogenesis, ovary, mitochondrial proteases

This abstract is being presented at: 3:15 PM in session:
SESSION 16: GENE EXPRESSION: TISSUE SPECIFICITY AND REGULATION OF STEROIDOGENESIS