Crowder and Surface Effects on Self-organization of Microtubules

Microtubules are an essential physical building block of cellular systems. They are organized using specific crosslinkers, motors, and influencers of nucleation and growth. With the addition of anti-parallel crosslinkers, microtubule pattern goes through transition from fan-like structures to homogeneous tactoid condensates in vitro. Tactoids are reminiscent of biological mitotic spindles, the cell division machinery. To accomplish these organizations, we use polymer crowding agents. Here we study how altering the properties of the crowders, such as size, concentration, and molecular weight, affect microtubule organization. Using simulations with experiments, we observe a scaling law associated with the fan-like patterns in the absence of crosslinkers. Tactoids formed in the presence of crosslinkers show variable length, depending on the crowders. The subtle differences correlate to individual filament contour length changes, likely due to effects on nucleation and growth of the microtubules. Using quantitative image analysis, we deduce that the tactoids differ from traditional liquid crystal organization, as they are limited in width irrespective of crowders and surfaces, and behave as solid-like condensates.