Subunit treadmilling of microtubules or actin in the presence of cellular barriers: possible conversion of chemical free energy into mechanical work.

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Abstract
Free microtubule or actin filaments, along with the monomeric forms of the protein, hydrolyze GTP or ATP to produce a flux of subunits through the polymer. This flux, called treadmilling, produces no useful work. In the cell, these filaments are likely to be constrained between nucleating sites and other barriers that will limit polymer growth. The effects of a small compression of the filaments resulting from polymerization against such barriers were studied. If subunits can still exchange at the 2 ends, treadmilling will occur here as well. Under these conditions the filament system can do useful work. The free energy of NTP hydrolysis can be used to transport materials, attached to the filament, against a resisting force. This process can in principle occur at high efficiency and bears a resemblance in a bioenergetic sense to the utilization of ATP free energy in muscle contraction. The same general principles apply to a polymer in which 1 end is anchored and 1 end is free. [Application of this equilibrium principle to sickle-cell Hb aggregation is discussed.].