MLN64 CONTAINS A DOMAIN WITH HOMOLOGY TO THE STEROIDOGENIC ACUTE REGULATORY PROTEIN (STAR) THAT STIMULATES STEROIDOGENESIS

MLN64 is a protein that is highly expressed in certain breast carcinom as, The C terminus of MLN64 shares significant homology with the stero idogenic acute regulatory protein (StAR), which plays a key role in st eroid hormone biosynthesis by enhancing the intramitochondrial translo cation of cholesterol to the cholesterol side-chain cleavage enzyme, W e tested the ability of MLN64 to stimulate steroidogenesis by using CO S-1 cells cotransfected with plasmids expressing the human cholesterol side-chain cleavage enzyme system and wild-type and mutant MLN64 prot eins, Wild-type MLN64 increased pregnenolone secretion in this system 2-fold, The steroidogenic activity of MLN64 was found to reside in the C terminus of the protein, because constructs from which the C-termin al StAR Homology domain was deleted had no steroidogenic activity, In contrast, removal of N-terminal sequences increased MLN64's steroidoge nesis-enhancing activity. MLN64 mRNA was found in many human tissues, including the placenta and brain, which synthesize steroid hormones bu t do not express StAR. Western blot analysis revealed the presence of lower molecular weight immunoreactive MLN64 species that contain the C -terminal sequences in human tissues, Homologs of both MLN64 and StAR were identified in Caenorhabditis elegans, indicating that the two pro teins are ancient, Mutations that inactivate StAR were correlated with amino acid residues that are identical or similar among StAR and MLN6 4, indicating that conserved motifs are important for steroidogenic ac tivity. We conclude that MLN64 stimulates steroidogenesis by virtue of its homology to StAR.