Structural plasticity of human Fascin1: A target for cancer treatment
Dr Jose Manuel Martin-Garcia [1]
Molecular Tour
The human protein Fascin1 is a cytoskeletal protein that binds actin filaments and cross-links them into compact parallel arrays that allow the formation of cellular structures for cell mobility and migration. Fascin1 is absent or present at minimal concentrations in adult human tissues, whereas it is overexpressed in tumors, in which it allows the spreading of cancer cells, thus contributing to metastasis. Therefore, Fascin1 is considered nowadays a novel and universal biomarker for aggressive human cancers with poor prognosis, and a target for the development of new antitumor therapies.
The cross-linking activity of Fascin1 is closely related to its structure and dynamic properties. Fascin1 is arranged in four β-trefoil-like domains (F1-F4) organized in two semi-independent lobes (F1-F2 and F3-F4). This structural organization facilitates coupling between domains and enables the accommodation of various inter-filament orientations, thereby bridging mismatches between the helical symmetry of F-actin and the hexagonal packing of the actin bundles. Two major acting-binding sites (ABS) have been identified: ABS1, consisting of the N- and C-terminal regions and the cleft between the F1 and F4 domains; and ABS2, located on the opposite side of the molecule and consisting of amino acids from the F1 and F2 domains. In addition to these, a third actin binding site (ABS3) formed by residues from the F3 domain has been proposed.
Our study suggests that the protein exists in various conformational substates prior to ligand binding, and this supports a mechanism of conformational selection. We have modelled for the first time the complete structure of the free wild-type human Fascin1 from the N-terminus to the C-terminus without interruptions and compared it with previously reported crystal structures. The rmsd. values for the alignment indicate structural similarities among all structures. However, the visual inspection shows larger differences in loop regions and solvent-exposed areas. β-trefoil domains 1 and 3, which have been reported to be somewhat interconnected, show the lowest rmsd. values, highlighting their resembling conformation. The high plasticity of Fascin1 is particularly evident in the molecule B of our structure, which captures larger conformational departures from previous structures.
To further investigate the flexibility of human fascin1, we carried out a structural alignment of the two molecules in the asymmetric unit and observed that even though the rmsd value indicates that the two molecules are similar (1.13 Å), a visual inspection of the superimposition shows that there are significant differences between them . These differences are mostly found in loop regions, especially in the regions comprising residues 49–60 in -trefoil domain 1, residues 274–281 in -trefoil domain 3 and residues 395–405 in -trefoil domain 4, which have been modeled in a different conformation in each molecule of the asymmetric unit (Fig. 4).
Fascin1 would only bind actin monomers if there is a proper spatial alignment of its major actin-binding sites. The structural differences between the ‘active’ and ‘inactive’ conformations of Fascin1 are subtle, limited to variations in loop and strand geometry that do not drastically alter the overall fold of the protein. Our results indicate that the protein`s structural flexibility is due to a salt bridge network established between its charged residues. These interactions introduce a certain rigidity into the protein that may or may not be beneficial for the actin binding activity. Specifically, we did not observe the presence of salt bridges between β-trefoil domains 1 and 4, that constitute the ABS1. This absence points to a dynamic region that requires flexibility for the initial contact with actin filaments.
References
- ↑ doi: https://dx.doi.org/10.1107/S2053230X25005254
