Just a protein out for a stroll

This animation of a “walking” protein inside a cell is going viral, and I became briefly obsessed with figuring out what’s going on here. Behavior that looks conscious and deliberate can’t, of course, be deliberate or conscious at the subcellular level. It’s just as fascinating, though.

What it is: A kinesin protein hauling a vesicle (pouch that can contain various things the cell needs) along a microtubule.

How are its “feet” moving? Reactions (see video, below.)The protein floats around till one of its feet hits a microtubule, where it binds. Then that foot releases a nucleotide (ADP), which leaves a site on its surface open that gets filled by a different nucleotide (ATP); this triggers the kinesin to wave its other foot forward. The front foot attaches and starts the same cycle; the back foot processes its ATP into ADP, releasing phosphate, and detaches from the tubule, then waves forward and “takes a step,” attaching to the microtubule again.

Why does it look deliberate? The process isn’t as smooth or clean as depicted in some animations like this, which are simplified to show the essential motions. In real life, there’s more jiggling and colliding. In fact, all the floating confusion is one reason why the “two-feet” method works — the kinesin is never completely detached from the microtubule, so it doesn’t float away.

Why is it moving in a particular direction? Microtubules have polarity, and some types of motor proteins will move toward the plus-end or minus-end.

I hat-tip @picpedant, where I first saw this, and recommend this by the artist who created the animation shown above, as well as this discussion of some of the misconceptions surrounding the viral version.