A novel function of “inactive” cytoplasmic ezrin in its closed conformation as an RNA binding protein to mediate osteosarcoma metastasis

Ezrin is a cytoplasmic protein that can exist in multiple conformational forms that are regulated by a post-translational modification. Phosphorylation at the threonine 567 residue confers an open form on ezrin by removing the head-to-tail interaction. Open ezrin was generally accepted as the active form because it can translocate to the plasma membrane and link the actin cytoskeleton to the membrane proteins. Closed ezrin, sequestered in the cytoplasm was considered inactive. Our previous work established that ezrin directly interacts with RNA binding proteins (RBPs), and here we show that ezrin expression correlates with cytoplasmic RBPs in human osteosarcoma samples. Therefore, we hypothesized that closed ezrin is not a dormant protein, but rather biologically active. Purified recombinant ezrin protein in its closed conformation directly bound RNA with 6.1 nM affinity. Ezrin binding specificity was highest for the guanine-rich sequences and RNAs with G-quadruplex (G4) secondary structures. Expression of closed ezrin rescued the transcriptome profiles better than open ezrin in an osteosarcoma cell line with its endogenous ezrin knocked out. Closed ezrin also rescued the metastatic ability of osteosarcoma xenographs in zebrafish. Taken together, our data established that the closed conformation of ezrin, previously thought to be inactive, can directly bind RNA and contribute to its metastatic phenotype.