Two dueling proteins help crimp the inner mitochondrial membrane into its familiar, accordion-like shape, Rabl et al. show.
The inner membrane of a mitochondrion is split into two parts. The peripheral portion, the inner boundary membrane, rests against a second membrane that encapsulates the organelle. Meanwhile the innermost portion, or cristae membrane, doubles back on itself again and again to form pleats, or cristae. A section of cristae membrane bends at two points—its tip and its base, where it connects to the inner boundary membrane. The tube-like structure that links the cristae and inner boundary membranes at this point is called a crista junction. How these connections form is a mystery.
To find out, Rabl et al. screened slow-growing yeast mutants, which often harbor malformed mitochondria. They pinpointed a strain that sported a faulty version of a protein they dubbed Fcj1. The protein embeds in the inner membrane and is particularly common at crista junctions, making it the first component of the junctions to be identified. Cranking up Fcj1's expression boosted the number of these junctions. When the team deleted Fcj1, by contrast, crista junctions vanished, and the cristae membrane distorted into parallel stacks of vesicles rather than a series of folds.
The researchers found that the ATP-making protein complex F1FO shows the opposite distribution to Fcj1—it amasses at cristae tips but is scarce at crista junctions. The two proteins appear to have an opposing function as well. Fcj1 prevents dimers of the synthase from zipping together. The researchers hypothesize that the curvature of the membrane at any point depends on the balance of Fcj1 and the ATP synthase. F1FO bends the membrane in one direction, whereas Fcj1 bends it the opposite way.