Discovery and evolution of mammalian Serpins as host-encoded viral protease inhibitors

About This Grant

PROJECT SUMMARY Viral proteases are essential for the replication of many RNA and DNA viruses. Through sequence specific recognition of cleavage sites, these proteases process viral polyproteins into their functional components, while also cleaving host proteins to facilitate viral replication. Interestingly, organisms from every domain of life encode proteins known as SERPINs (SERine Protease INhibitors) that serve as natural inhibitors of serine and cysteine proteases, the types of proteases encoded by many viruses. SERPINs contain a conserved structural core and a disordered reactive center loop (RCL) that ‘baits’ a protease into cleaving it by mimicking the sequence specificity of the protease to be inhibited. Following protease-mediated cleavage of the RCL, the SERPIN undergoes a large conformation change that covalently traps and inactivates the protease, thus connecting sequence-specific recognition of the SERPIN RCL to protease inhibition. While SERPINs have been largely characterized as inhibitors of a wide range of cellular proteases, we hypothesize in this grant that SERPINs have also evolved, in a species-specific manner, to directly inhibit viral protease activity and thereby inhibit viral replication. Supporting this hypothesis, we have found that several mammalian SERPINs contain RCLs that are rapidly evolving under positive selection, consistent with SERPINs being engaged in evolutionary arms races with pathogen-encoded proteases. Moreover, we have identified primate SERPINs that have independently evolved sequences that mimic the cleavage site preferences of proteases from picornaviruses and coronaviruses, and find that these SERPINs can inhibit viral protease activity during infection. These data lead us to propose a model in which SERPINs, with their protease baiting RCLs, represent a modular platform for evolution of host-specific antiviral activity through viral protease inhibition. Based on this model, we now propose to use evolutionary biology, protease biochemistry, and virology to discover and characterize the natural variation, evolvability, protease specificity, and antiviral activity of viral protease inhibitors among mammalian SERPINs. In Aim 1, we will test the antiviral potency and specificity of primate SERPIN-mediated viral protease inhibition, while also computationally and functionally searching for additional mammalian SERPINs that can inhibit proteases from a wide range of RNA viruses. In Aim 2, we will probe the modularity and evolvability of SERPINs to determine the degree to which the SERPIN ‘core’ and RCL sequence impact function, and how sequence evolution of each can mediate selective viral protease inhibition. By identifying SERPINs as novel, rapidly evolving, host-encoded viral protease inhibitors, our work will reveal the impact of SERPIN and viral protease evolution on this new host-virus evolutionary conflict and on species-specific barriers to virus replication, and unveil the evolutionary potential of SERPINs to inhibit a wide range of viral proteases.