Direct Measurement of the Pressure Generated by a 1D Protein Gas Confined within Microtubule Overlaps

Abstract

The integrity of the mitotic spindle during anaphase is facilitated by antiparallel microtubule-microtubule overlaps in the spindle midzone. Within these overlaps, motor proteins (e.g. kinesin-5, kinesin-14) as well as passive, non-enzymatic microtubule crosslinkers (e.g. from the MAP65 family) localize and influence the mechanical stability of the overlaps. Here, we show in vitro that the diffusible microtubule crosslinker Ase1, a member of the MAP65 family, can slow down and halt the shortening of microtubule-microtubule overlaps driven by the kinesin-14 Ncd. Using mathematical modeling we show that Ase1 confined in a microtubule overlap behaves like a 1D gas upon compression, i.e. producing an entropic force opposing the compression. Direct measurement of the entropic force by optical tweezers yielded values in the pN-range, comparable to the forces produced by motor proteins present in the spindle midzone. We hypothesize that entropic pressure may be a general mechanism of force production in biological systems.