Restriction of HIV and coronavirus infections by the innate immunity protein Shiftless

Programmed ribosomal frameshifting (PRF) is a mechanism of recoding that allows synthesis of multiple proteins from the same mRNA by shifting the translation reading frame. Many viruses, including medically important HIV and coronaviruses rely on PRF to increase their coding capacity and modulate appropriate stoichiometric ratios of viral proteins. An interferon-inducible restriction factor Shiftless (SFL) can block the frameshifting required for viral translation and infectivity. Understanding the mechanism of action of SFL will undoubtedly guide the design of new antiviral therapeutics. However, most structural and functional aspects of SFL and its role in viral infections are unknown. We intend to characterise the role of SFL in HIV and coronavirus infections using a three-pronged approach: (i) We will elucidate the mechanism of inhibition of PRF on viral mRNAs by SFL using a state-of-the-art in vitro reconstituted translation system and depletion/deletion of SFL in human cell lines. (ii) We will determine the interaction of SFL, translating ribosome and frameshifting viral mRNAs using cryo-electron microscopy. (iii) Based on the structural data obtained, we will generate mutants of SFL and characterise their activities by in vitro and cellular translation assays to identify specific domains and amino acid residues required for the antiviral activity of SFL. The results expected from our proposed study should not only be crucial for understanding the molecular mechanism of SFL but should also provide vital inputs to the development of antiviral therapeutic agents by either mimicking or upregulating SFL expression against critically important viruses such as HIV and SARS-CoV-2. Importantly, our research should continue to be relevant for the treatment of any future frameshifting virus.