The unphosphorylated, closed form of ezrin binds to RNA to maintain a metastatic phenotype in osteosarcoma cells

Abstract

Ezrin is a cytoplasmic protein that can exist in multiple conformations that are regulated by phosphorylation at Thr⁵⁶⁷. The phosphorylated, open form of ezrin generally has been considered the active form because it translocates to the plasma membrane. In contrast, the unphosphorylated, closed form of ezrin is sequestered in the cytoplasm and is considered inactive, although it directly interacts with cytoplasmic RNA binding proteins. Here, we found that the closed form of ezrin is itself an RNA binding protein with biological activity. The abundance of ezrin correlated with that of RBPs in human osteosarcoma samples. Purified recombinant ezrin protein engineered to maintain a closed conformation (rEZRIN-T567A) directly bound RNA, with greatest affinity for guanine-rich sequences and RNA G-quadruplexes (G4 RNAs). Expressing closed ezrin in ezrin-null osteosarcoma cells restored the transcriptomic and proteomic profiles. Closed ezrin bound to endogenous mRNAs associated with pathways related to RNA processing and splicing, DNA maintenance, and cellular metabolism. In zebrafish, expression of closed ezrin rescued the metastatic capability of ezrin-null osteosarcoma xenografts. Our findings demonstrate that the closed conformation of ezrin—previously thought to be inactive—can directly bind RNA, regulate transcription and translation, and contribute to a metastatic phenotype in osteosarcoma cells.