When we think about cell division, we usually focus on the chromosomes and how they separate into the daughter cells. Actually, many other organelles in the cytoplasm need to duplicate themselves, too. How do you duplicate a power plant — a mitochondrion?

Consider that each mitochondrion (and there are many in most eukaryotic cells) has dozens of those spinning motors called ATP synthase in their membranes. Several other complex machines lead up to ATP production. These machines run constantly. When it’s time to divide, all the parts of a working mitochondrion have to be duplicated on cue, at the right time, in the right sequence, with quality control — otherwise serious diseases can result.

Additional insight into mitochondrion duplication comes in a review article in Current Biology by three microbiologists from Monash University in Australia. They obviously can’t describe everything that goes on, because it is far too complicated. It’s also poorly understood. Instead, they focus on the membrane transporters whimsically known as TIM and TOM: transporter inner membrane and transporter outer membrane. TIM and TOM are actually large families of proteins that form complexes. Their job is to control what goes in and out of the organelle’s two-fold membranes.

Mitochondria cannot be made de novo, so pre-existing mitochondria must be inherited at each cell division. A new study demonstrates cell-cycle-dependent regulation of the activity of the TOM translocase complex to induce mitochondrial biogenesis during the M phase of the cell cycle.

In all eukaryotes, pre-existing mitochondria are used as templates to build more mitochondrial mass, ahead of cell division or in response to increased metabolic demand. Mitochondrial biogenesis requires the import of up to 1,000 different proteins into the organelle, and all proteins imported across the outer mitochondrial membrane enter through a protein translocase called the TOM complex. This complex is a remarkable nanomachine composed of a core formed by the ?-barrel channel Tom40 and additional subunits, each of which has a single ?-helical transmembrane segment. [Emphasis added.]

Most of the proteins have to be translated from genes in the nucleus. Only about 13 proteins the organelle needs are coded in mitochondrial DNA. The majority of those thousand proteins, therefore, are going to have to work with TIM and TOM on their way in. Wikipedia describes the process:

The vast majority of proteins destined for the mitochondria are encoded in the nucleus and synthesized in the cytoplasm. These are tagged by an N-terminal signal sequence. Following transport through the cytosol from the nucleus, the signal sequence is recognized by a receptor protein in the transporter outer membrane (TOM) complex. The signal sequence and adjacent portions of the polypeptide chain are inserted in the TOM complex, then begin interaction with a transporter inner membrane (TIM) complex, which are hypothesized to be transiently linked at sites of close contact between the two membranes. The signal sequence is then translocated into the matrix in a process that requires an electrochemical hydrogen ion gradient across the inner membrane. Mitochondrial Hsp70 binds to regions of the polypeptide chain and maintains it in an unfolded state as it moves into the matrix.

The proteins are not folded until they get inside the new mitochondrion. Then, other complex processes assemble them into the machines that will begin operation when duplication is complete.

One of those machines, a proton pump named Mitochondrial Complex 1, was recently described by Science Magazine as "among the largest and most complicated membrane protein complexes." It has to be built correctly, because "Its malfunctions are implicated in many hereditary and degenerative disorders." All this construction must take place before cell division can complete.

What the microbiologists from Monash found, working with yeast cells, was that there’s a checkpoint mechanism that "turns on" the TOM gates at a certain phase of the cell cycle. It also revs up translation of mitochondrial genes in the nucleus. The master regulator of mitosis, an enzyme known as CDK1, puts a phosphate tag on Tom6, one of the TOM proteins. Here’s a taste of the interactions involved among the players:

Harbauer et al. developed biochemical assays to directly demonstrate that the phosphorylation of Tom6 is mediated by CDK1, and that phosphorylated Tom6 is assembled into the mitochondrial outer membrane with increased efficiency (Figure 1). What’s more, the increased levels of Tom6 serve to promote an increased assembly of Tom40 into the outer membrane: in this regard, the steady-state level of Tom6 serves as a rheostat to ‘dial-up’ or ‘dial-down’ the amount of TOM complex carried by mitochondria. This rheostat is in turn under the control of the cyclin Clb3, which is the specific trigger to activate CDK1 to phosphorylate Ser16 of Tom6. Clb3 expression is tightly controlled to be active only in M phase.

You get the picture. A checkpoint in the cell cycle turns on this protein, which controls that protein, which controls another protein. A series of switches control the timing and activity of duplication of this complex organelle. These switches are sensitive to feedback to ensure everything happens on time, in the right order.

The authors are in awe of how efficient this is. It wouldn’t make sense for the protein machines to go to so much work to build another power plant for no good reason.

Now, the stage is set to approach a detailed, systems-level understanding of ‘why’ and ‘how’ mitochondrial biogenesis is kicked into top gear. This matters in cell biology. The primary function of mitochondria in most organisms is to maximize the returns in energy currency from investments made in securing carbon from the environment. Via the action of mitochondria, carbon derived from sugars and fats can be used to produce a maximal amount of ATP, but, at a systems level, there is a great paradox in the use of this organelle. To translate the thousand or so proteins needed to build mitochondrial mass and to transport, fold and assemble these proteins into a mitochondrion costs an enormous amount of ATP. It is also costly, in term of ATP consumption, to replace the electron-transport complexes burnt out by oxidative damage in the course of ATP production, and to replicate the mitochondrial DNA and transcribe the mitochondrial RNAs that encode several of the electron-transport proteins. It is little wonder then that the amount of mitochondrial mass accumulated in each round of the cell cycle would be judiciously controlled so as to be just sufficient to ensure maximum dividends to the cell’s energy investments.

So how did this all evolve? They don’t say. Their only mention of evolution is this one passing reference, offered without evidence or proof:

Four of the subunits surrounding the Tom40 ?-barrel — Tom5, Tom6, Tom7 and Tom22 — are tail-anchored proteins. For Tom7 and Tom22 it is certain that they were present in the ancestral TOM complex, early in the evolution of mitochondria.

Why that is so certain, again, they don’t say. Perhaps without that pair of subunits, the complex wouldn’t work, and eukaryotic life would never have gotten started. (It should be noted that prokaryotes, though they lack mitochondria, use the same machinery for ATP production.) Since failure of these machines causes serious disease, we can safely infer that the transporter mechanisms, along with their signals and links to the cell cycle, are irreducibly complex.

Illustrators and reporters often portray these systems in oversimplified ways, as if they somehow "pop" into existence by evolution. This brief look into the window of a real mitochondrion helps you appreciate the sophistication of intelligent design in even a tiny yeast cell.

Image: Athlone Power Station, Cape Town, South Africa, by Simisa (Own work Simisa (talk�� contribs)) [CC BY-SA 3.0], via Wikimedia Commons.