Murine RAX (PKR Activator X) protein and its human ortholog PACT (Protein Activator of Interferon-induced Protein Kinase) are the only known cellular activators of the interferon-induced dsRNA-dependent protein kinase, PKR. RAX and PACT are 98% identical in amino acid sequence. In addition, RAX/PACT associates with Dicer to form the RNA-induced silencing complex (RISC) and promote small RNA-mediated gene silencing. RAX is phosphorylated on serine 18 during cellular stress applications and this phosphorylation is required for PKR activation and consequent translation inhibition during events such as growth factor deprivation, cytotoxic cytokine or chemotherapy treatment and viral infection. In addition, exogenous expression of RAX(S18A) promotes anchorage-independent growth of fibroblast cell lines, indicating that RAX phosphorylation is required for normal cellular growth regulation. A peptide composed of RAX amino acids 13 to 25, where phosphoserine was substituted for serine at position 18, was used as antigen to generate a hybridoma cell line that produces monoclonal antibody specific to phosphorylated RAX.
