Single molecule tracking fluorescence microscopy in mitochondria reveals highly dynamic but confined movement of Tom40

Most of the mitochondrial proteins are imported from the cytoplasm, with only a small fraction (about 1%) encoded in the mitochondrial genome. Import is mediated by two complexes: TOM (transporter outer membrane) and TIM (transporter inner membrane). We have a pretty good idea about the players involved in mitochondrial protein import, but we have very little idea about the dynamics of TOM/TIM movement in the mitochondrial membrane.

We tried addressing this question using single molecule fluorescent microscopy in isolated yeast mitochondria. What we see is that Tom40, the central component of TOM complex, is highly confined (i.e. restricted in terms of aerea it can sample) but within its confinement it moves pretty rapidly.

References:

Kuzmenko et al., Scientific Reports (2011) 1:95

Regulation of mitochondrial protein transport

Mitochondria have their own genome, own translational machinery, own ribosomes, but still, most of the proteins they import from the cytosole. And this they do using two protein complexes in the outer and inner membranes: TOM (Transporter Outer Membrane) and TIM (Transporter Inner Membrane). TOM itself consists of several subunits: Tom40 forms a pore through which proteins get transported, Tom20 and Tom70 work as receptors recognizing the mitochondrial proteins in the cytoplasm, and several more proteins helping out.

TOM and TIM, figure lifted from Chacinska at al., 2009

And now joint effort of Pfanner and Meisinger labs lead to a discovery that in yeast TOM-mediated protein transport is regulated by kinases casein kinase 2 (CK2) and protein kinase A (PKA). CK2 promotes TOM biogenesis, and PKA phosphorylates Tom70 component of TOM under nonrespiring conditions, inhibiting it. This finding basically opens a new field: regulation of mitochondrial protein transport. Just like that.

References:

Chacinska A, Koehler CM, Milenkovic D, Lithgow T, & Pfanner N (2009). Importing mitochondrial proteins: machineries and mechanisms. Cell, 138 (4), 628-44 PMID: 19703392

Schmidt O, Harbauer AB, Rao S, Eyrich B, Zahedi RP, Stojanovski D, Schönfisch B, Guiard B, Sickmann A, Pfanner N, & Meisinger C (2011). Regulation of mitochondrial protein import by cytosolic kinases. Cell, 144 (2), 227-39 PMID: 21215441