The microtubule-associated protein RIC1 forms biomolecular condensates to promote the polymerization of microtubule bundles in vitro

Résumé fourni par la source

The Arabidopsis thaliana RIC1, a key member of the ROP (Rho GTPase of Plants)-Interactive CRIB motif-containing protein family, has been characterized as a plant-specific microtubule-associated protein. RIC1 is thought to function as a key downstream effector of the small GTPase Rho-of-Plant 6 (ROP6), orchestrating the formation of well-ordered cortical microtubule arrays and promoting the interdigitated growth pattern of cotyledon or leaf epidermal pavement cells. However, the precise physicochemical properties governing RIC1's function in microtubule organization regulation remain known. Here, we demonstrate that RIC1 is a predicted intrinsically disordered protein that forms biomolecular condensates, which promote the polymerization of microtubule bundles in vitro. In Arabidopsis cotyledon pavement cells, RIC1 forms distinct condensate-like puncta, with a specific subset associating directly with cortical microtubules. RIC1 self-assembles into distinct spherical condensates under in vitro macromolecular crowding conditions. Remarkably, these RIC1 condensates co-condense with tubulin in vitro. We further demonstrate that RIC1 condensates elevate the local tubulin concentration, correlate with enhanced microtubule nucleation, and promote the polymerization of microtubule bundles in vitro. Moreover, the growing microtubule bundles actively reshape these condensates. Thus, RIC1 condensates may provide a specialized microenvironment that enables tubulin sequestration while simultaneously regulating microtubule nucleation and polymerization in vitro. This study provides crucial insights into the biomolecular condensation properties of the microtubule-associated protein RIC1, establishing a groundwork for future investigations into its in vivo functional mechanisms.

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