We have just seen that bacterial cells were preadapted for the formation of mitochondria. More specifically, one out of every twenty bacterial proteins harbors the equivalent of the mitochondrial targeting sequence which is needed for entry into the mitochondria. But this story gets more interesting. To appreciate the twist that comes next, let’s step back to make sure we can visualize the process of transporting mitochondrial proteins into the mitochondria.
First, like gram negative bacteria, the mitochondria have two membranes:
We want to focus on the outer membrane, as this is where the mitochondrial proteins, synthesized by the ribosomes in the cell’s cytoplasm, must be targeted for entry into the mitochondria.
Mitochondrial proteins are sent to something called the TOM complex that is found in the outer membrane. The figure below shows a simplified version of this complex that is composed of a receptor, that binds the mitochondrial targeting sequence on mitochondrial proteins, and shuttles it to the channel for entry into the mitochondria (TIM is the inner membrane transporter).
The figure below provides more detail:
You can see the mitochondrial protein on the top with its exposed mitochondrial targeting sequence (the + signs near the N). The mitochondrial targeting sequence binds to the receptor (R) which passes the protein to the TOM channel (shown as GIP in this picture). The protein is then threaded through both TOM and TIM, using the same proton gradient that mitochondria generate for their ATP synthases.
Now, take a look at a more detailed figure:
As you can see, TOM is composed of multiple proteins. Tom7, Tom22, and Tom40 are the core elements of this complex, where Tom40 form the pore itself and is likely to be the descendent of bacterial porins (protein pores found in the outer membranes of gram negative bacteria).
It turns out that the mitochondrial targeting sequence interacts with multiple components of this system:
The intracellular sorting of newly synthesized precursor proteins (preproteins) to mitochondria depends on the “mitochondria-targeting sequence” (MTS), which is located at the amino termini of the preproteins. MTS is required, however, not only for targeting newly synthesized preproteins to mitochondria, but also for all the following steps along the mitochondrial protein import pathway. MTS of nascent preproteins is first recognized by a cytoplasmic molecular chaperone, MSF, and then by Tom70 and Tom20 of the mitochondrial outer membrane receptor complex, Tom5 and Tom40 of the outer membrane protein translocation machinery, Tim23 of the inner membrane protein translocation machinery, and finally the processing peptidase, MPP, in the matrix. MTS is a multi-role sorting sequence which specifically interacts with various components along the mitochondrial protein import pathway. Recognition of MTS at multiple steps during the import of preproteins may contribute to the strict sorting of proteins destined for mitochondria.
From: Omura, T. 1998. Mitochondria-Targeting Sequence, a Multi-Role Sorting Sequence Recognized at All Steps of Protein Import into Mitochondria J. Biochem 123: 1010-1016.
The player I want to focus on is Tom20. Why? Because Tom20 is the receptor protein that snags the mitochondria targeting sequence. Tom20 is not essential, as yeast strains have been constructed where Tom20 was removed and the yeast remain viable. This means that the receptor simply enhances the efficiency of the transport process.
It’s time for the story to get more interesting….