Calcium and Sodium-mediated Dynamic Assembly of Intermediate Filament-like Protein FilP

Background: Cytoskeletal elements play key roles in cell morphology, cell division, cell mobility, and DNA partitioning in all domains of life. The IF-like protein FilP was discovered in Streptomyces coelicolor, and it was found to perform a structurally important cytoskeletal role by providing direct mechanical support for the cells. Objective: This work investigated the factors influencing FilP polymerization under a variety of conditions. Methods: DLS technique was applied to real-time monitor the in vitro assembly process of Streptomyces coelicolor FilP. Results: The presence of small amounts of divalent cations, such as CaCl2 or MgCl2, enhanced the polymerization of FilP, while higher amounts suppressed its polymerization. Moreover, high concentrations of NaCl, KCl, NH4Cl, and KNO3 both suppressed the polymerization of FilP. EDTA was found to have a very prohibitive effect on FilP polymerization, and even the following addition of Ca²⁺ could not initiate the assembly of FilP. FilP polymerized under a range of pHs ranging from pH 6 to pH 8, while the polymerization degree was sensitive to pH. FilP formed network-like, striated filaments at neutral pH, while the filaments became more disordered or loosely packed at pH 8 and pH 6, respectively. Conclusion: FilP assembly is calcium-mediated. Ca²⁺ is not only required for FilP polymerization, but also required for FilP to maintain the higher-order polymer structure. The accelerative effect of Ca²⁺ and the suppressive effect of Na⁺ persisted under a wide range of conditions, suggesting that FilP might use calcium and sodium ions as a general mechanism to mediate its polymerization process.