S100 Proteins Control Cytoskeletal Dynamics in Cancer

The S100 family of calcium binding proteins exhibits a unique pattern of cell type specific expression. These proteins are found in the cytoplasm and/or nucleus of a variety of cells, and involved in the control of a wide range of cellular processes such as cell cycle progression and differentiation. S100A4 and S100A6 are members of the S100 protein family that interact with several molecular targets including the heavy chain of non-muscle myosin IIA (NM IIA) and annexin II, respectively. NM IIA is a major actin-associated motor protein, which is involved in cell motility and cytokinesis. Assembly/disassembly of myosin filaments is primarily controlled by myosin light chain phosphorylation. However, small calcium-binding proteins of the S100 family also play an active role in the dynamics of actin-myosin filaments, leading to an increase in the dissemination of tumour cells. Accordingly, the main aim of this work was to study the molecular mechanism underlying S100A4/A6 function in epithelial mesenchymal transition (EMT) and provide in vivo data highlighting their role in the regulation of myosin dynamics. Intriguingly, we employed a novel transition electron microscopy approach to study the function of non-muscle IIA isoforms and their interactions with S100A4/A6 in A431/ZEB2 cells undergoing an EMT. Our data confirmed that both 6S and 10S myosin isoforms do exist in cells and directly interact with S100A4/A6 in vivo. Depletion of S100A4 resulted in the disappearance of the peaks corresponding to monomeric myosin indicating that S100A4 is required for balancing monomer-polymer equilibrium in cells. In blot overlay, both S100A4 and S100A6 showed similar binding site on myosin fragment 4 (C-terminus). However, a new S100A6 binding site was mapped on myosin heavy chain represented in M53 fragment which is a part of rod domain. In addition to the solubility of myosin in high ionic buffer, S100A4 and S100A6 are able to solubilise the myosin which was measured by the turbidity assay. Moreover, a decrease in ATPase activity of actomyosin complex in cells undergoing EMT was observed in the presence of S100A4/A6. In conclusion: This study shows that S100A4/A6 protein interacts with NM IIA. There is no redundancy and both proteins promote myosin dynamics, cell migration and invasion. S100A4 and S100A6 are up-regulated by ZEB2 and is implicated in the dynamic regulation of myosin filaments by switching the balance towards monomeric myosin.