Can microtubule diffusiophoresis be present during spindle formation?

Microtubules (MT) assemble the mitotic spindle during mitosis in eukaryotic cells. The concentration gradient of RanGTP plays an important role during this process. As microtubules are highly negatively charged and concentration gradients of various molecules are ubiquitous in cells, we asked whether diffusiophoresis of microtubules can be present in such an environment. Diffusiophoresis is the spontaneous motion of particles under a concentration gradient of solutes. In this study, we demonstrate migration of MTs in assembly buffer under MgCl₂ gradients and quantify the diffusiophoretic behavior by comparison with multi-ion diffusiophoresis and diffusioosmosis models. We further study the diffusiophoresis of MTs under various concentration gradients of relevant biological solutes (ATP, GTP, and RanQ69L) and unravel diffusiophoresis in more complex environments. We find that the long-range interaction between some proteins and MTs induces diffusiophoresis of MTs along a concentration gradient, and by using Xenopus laevis egg extract we show that such a mechanism of protein-driven diffusiophoresis is possible in active cytoplasmic environments.