Kinessa

Kinessa walks a microtubule tightrope with a cargo sphere on her back, one ATP per stride, hand over hand.

The Physics

Kinesin-1 is a two-headed motor protein that carries cargo along microtubules. Using an optical trap, Svoboda, Schmidt, Schnapp and Block saw discrete 8 nm steps of the molecule's centre of mass. Yildiz and colleagues labelled one head and saw it move in alternating steps of about 17 nm and 0 nm: the heads swap places, hand over hand.

With a force clamp, Visscher, Schnitzer and Block (1999) found one ATP hydrolysed per 8 nm step and a stall force that depends on ATP concentration. Summarising those measurements, Schnitzer, Visscher and Block (2000) put the motor's maximum load at about 5.5–7.5 pN.

W = F × d = 6 pN × 8 nm = 48 pN·nm ≈ 12 k_BT

• W: work per step (pN·nm)
• F: load force, near stall (pN)
• d: step size (nm)
• k_BT: thermal energy, 4.1 pN·nm at 298 K

Svoboda et al., Nature 365, 721 (1993); Visscher et al., Nature 400, 184 (1999); Schnitzer et al., Nat. Cell Biol. 2, 718 (2000); Yildiz et al., Science 303, 676 (2004).

Honest caveat: The stall force is not always 6 pN; it depends on conditions. 48/4.1 is 11.7, so “about 12 k_BT”.

On the shirt

Front: Kinessa, the pixel-art character, with “8 NM PER STEP”. Back: the one-ATP step, the stall force, the work per step in k_BT and the 1993, 1999 and 2004 sources. Printed on a unisex tee.