The requirement of phosphorylation on a threonine residue in the acute regulation of steroidogenesis in MA-10 mouse Leydig cells.

In the present study we have used several non-phosphorylatable analogs of the amino acids threonine and serine to determine the role of phosphorylation in the acute regulation of steroidogenesis in MA-10 mouse Leydig tumor cells. Our results indicate that substitution of the threonine analog into protein results in a inhibition of hormone stimulated steroid production in these cells while none of the serine analogs employed displayed a similar inhibition. Strikingly, only the threonine analog resulted in the inhibition of the synthesis of several 30 kDa mitochondrial proteins which we have previously shown to be induced by hormone stimulation of MA-10 cells. Thus, it is apparent that phosphorylation of a threonine residue is obligatory for the acute production of steroids in MA-10 Leydig cells and also for the synthesis of a series of previously described mitochondrial proteins. However, a causal relationship between the 30 kDa mitochondrial proteins and steroid regulation cannot be made unequivocally at this time.