Figure 1.

Schematic representation of two proposed mechanisms for the acute regulation of steroidogenesis by StAR protein. Following trophic hormone binding to a cell surface receptor, cAMP increases and activates protein kinase A. Protein kinase A increases the synthesis of StAR and its phosphorylation giving rise to pp37. In proposal A, StAR is imported into the mitochondria and subsequently processed to the mature pp30. During the import process, it is proposed (8) that cholesterol is transported to the inner mitochondrial membranes. Proposal B1 takes into account the data of Arakane et al. (2), which suggests that the interaction of the StAR protein with the mitochondrial surface leads to cholesterol movement. Further, it is proposed (B2) that the importation of StAR and cleavage to pp30 is a mechanism of inactivating cholesterol flux into the mitochondria.