The Harvard multimedia team that put together that pretty video of the Inner Life of the Cell has a whole collection of videos online (including Inner Life with a good narration.) Go watch the one titled F1-F0 ATPase; it’s a beautiful example of a highly efficient molecular motor, and it’s the kind of thing the creationists go ga-ga over. It’s complex, and it does the same rotary motion that the bacterial flagellum does; it has a little turbine in the membrane, a stream of protons drives rotation of an axle, and the movement of that axle drives conformation changes in the surrounding protein that promote the synthesis of ATP. It’s a molecular machine all right. Makes a fellow wonder if possibly it’s “irreducible”, doesn’t it?
Well, it’s not. It can be broken down further and it still retain that rotary motion.
First, let’s look at some stills from the Harvard movie. This is the overall structure of the complex; the blue and green parts are the α and β subunits, which are responsible for the actual ATPase activity.
In this next image, the α and β subunits have been removed to expose the γ subunit, in orange, which is the axle of the whole complex. Put α and β together with γ and some ATP, and the α and β break down the phosphates and set γ to spinning. This can even be visualized; anchor the α and β proteins to a glass slide, attach some 40-nm gold beads to the γ subunit so that you’ve got something large enough to see, and you can make movies of the motion, study rotational kinetics, etc. Furuike et al. have described exactly that in a recent issue of Science.
The researchers took another step, though, one that was intended to study further details of the molecular interactions involved in ATPase activity, but which also tells us something about the evolution of this motor and undercuts creationist claims of “irreducible complexity”. They created modified versions of the proteins; in particular, they made various truncated forms of the γ subunit, the axle, that eliminated the axle structure. A wheel without an axle? What good is that?
This is an illustration of their modified γ proteins. It’s upside down relative to the Harvard movie, unfortunately, but the color scheme is the same — the axle is red and orange, while α and β are blue and green. In D is an axle with just the tip deleted; in F, the whole thing is gone, and we just have the base of the γ protein bouncing about on top of the α and β subunits.
Now the surprise: these axle-less mutants still spin! Even the stumpy one that has no axle at all. And the rotation is in the correct direction. The authors note that this is a rather substantial difference from man-made machines.
Most manmade rotary machines rely on a rigid axle held by a static bearing: thanks to the constraint exerted by the bearing, almost any force acting on the axle is converted to a torque through a lever action. Nature seems to have adopted this simple principle for the bacterial flagellar motor and the proton-driven Fo motor of the ATP synthase: Their rotor axis is held stationary by stator bearings, and thus force from one driving unit, acting on only one point on the rotor, suffices to produce torque. For the short γ mutants here, however, the concept of a rigid axle in a static bearing no longer applies. Yet the mutants do rotate.
What was also observed, though, is that the efficiency is greatly reduced: rotation is more halting, and the likelihood of the γ subunit actually binding to the α/β complex is much lower. The more complete the axle, the more reliable the rotation became. This is obviously what we’d expect from evolution: that a molecular machine would not be an all-or-nothing affair, but that incremental variations would produce incremental changes in efficiency.
Furuike S, Hossain MD, Maki Y, Adachi K, Suzuki T, Kohori A, Itoh H, Yoshida M, Kinosita K Jr. (2008) Axle-less F1-ATPase rotates in the correct direction. Science 319(5865):955-8.