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Characterization of the phosphoprotein of human metapneumovirus and its role in formation of viral liquid organelles

Human metapneumovirus (HMPV) is a recently identified RNA virus that frequently causes lower respiratory tract infections in infants, the elderly, and the immunocompromised, yet it has no FDA approved antivirals or vaccines. Understanding how HMPV replicates could help identify targets for development of new therapeutics. A key aspect of the viral replication cycle of HMPV is the formation of membrane‐less, liquid‐like replication and transcription centers termed inclusion bodies (IBs). A major component of IBs is the tetrameric HMPV phosphoprotein (P), which contains N and C terminal domains that lack defined secondary structures, called intrinsically disordered regions (IDRs). IDRs are thought to be involved in formation of liquid organelles through a process called liquid‐liquid phase separation (LLPS). However, the role P plays in IB formation and function is not understood. Our overall hypothesis is that the IDRs and post‐translational modifications of the phosphoprotein allow it to facilitate LLPS for IB formation. Western blots of cell lysate show two distinct bands for HMPV P, both of which contain phosphorylated protein and are detectable with either C terminal or N terminal HA tags. However, a C terminal HA tag disrupts IBs when co‐transfected with N, suggesting interactions through the C terminal domain are critical for IB formation. Purified P can form liquid‐like droplets without the presence of N, indicating it plays an important role in LLPS. Future studies will identify which domains of P are critical for LLPS and how phosphorylation and other modifications of P contribute to IB formation.