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

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.

This work investigated the factors influencing FilP polymerization under a variety of conditions.

DLS technique was applied to real-time monitor the in vitro assembly process of Streptomyces coelicolor FilP.

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.

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.