Mitochondrial protein import and the genesis of steroidogenic mitochondria

Abstract

The principal site of regulation of steroid hormone biosynthesis is the transfer of cholesterol from the outer to inner mitochondrial membrane. Hormonal stimulation of steroidogenic cells promotes this mitochondrial lipid import through a multi-protein complex, termed the transduceosome, spanning the two membranes. The transduceosome complex is assembled from multiple proteins, such as the steroidogenic acute regulatory (STAR) protein and translocator protein (TSPO), and requires their targeting to the mitochondria for transduceosome function. The vast majority of mitochondrial proteins, including those participating in cholesterol import, are encoded in the nucleus. Their subsequent mitochondrial incorporation is performed through a series of protein import machineries located in the outer and inner mitochondrial membranes. Here we review our current knowledge of the mitochondrial cholesterol import machinery of the transduceosome. This is complemented with descriptions of mitochondrial protein import machineries and mechanisms by which these machineries assemble the transduceosome in steroidogenic mitochondria.

Figure 1. Transduceosome formation at the mitochondria upon hormonal stimulation

Cytosolic transduceosome proteins PKA, ACBD3, and ACBD1 are shown in blue while mitochondrial matrix proteins STAR, CYP11A1, Adx, and AdxR are shown in purple. OMM transduceosome proteins VDAC and TSPO, and the IMM protein ANT, are represented in brown.

Figure 2. Mechanisms of protein import into the OMM

Import of α-helical proteins into the OMM requires the TOM complex, shown in orange, though exact mechanisms vary by targeting mechanisms located on the protein. Import of β-barrel proteins into the OMM requires both the TOM complex and the SAM complex, shown in blue. Transport of the preprotein from the TOM complex to the SAM complex uses the Tim8-Tim13 chaperone complex, shown in green.

Figure 3. Mechanisms of protein import into the IMM

Alpha-helical IMM proteins are imported via Tim23, shown in purple. Import of matrix proteins requires Tim23 and the additional PAM complex and mtHSP70 (blue). Import of multi-membrane spanning IMM proteins is accomplished through the small Tim proteins Tim9-Tim10, shown in green, which transport the preprotein to Tim22, shown in pink. TOM complex is shown in brown, Tim23 complex is shown in purple, and Tim22 complex is shown in pink.

Figure 4. STAR protein import and its relationship to the transduceosome in a working model of OMM proteins

Experimental evidence demonstrates that TSPO and VDAC are critical for the mitochondrial import and processing of the cytoplasmic STAR protein. Though the mitochondrial TOM (faded orange) and Tim complexes (not shown) are implicated in STAR import by its mitochondrial localization and presequence tag, direct experimental evidence of association with the mitochondrial protein import machinery is lacking. See text for further details.