Analysis of Group IV Viral SSHHPS Using In Vitro and In Silico Methods

Alphaviral enzymes are synthesized in a single polypeptide. The nonstructural polyprotein (nsP) is processed by its nsP2 cysteine protease to produce active enzymes essential for viral replication. Viral proteases are highly specific and recognize conserved cleavage site motif sequences (~6-8 amino acids). In several Group IV viruses, the nsP protease(s) cleavage site motif sequences can be found in specific host proteins involved in generating the innate immune responses and, in some cases, the targeted proteins appear to be linked to the virus-induced phenotype. These viruses utilize short stretches of homologous host-pathogen protein sequences (SSHHPS) for targeted destruction of host proteins. To identify SSHHPS the viral protease cleavage site motif sequences can be inputted into BLAST and the host genome(s) can be searched. Cleavage initially can be tested using the purified nsP viral protease and fluorescence resonance energy transfer (FRET) substrates made in E. coli. The FRET substrates contain cyan and yellow fluorescent protein and the cleavage site sequence (CFP-sequence-YFP). This protease assay can be used continuously in a plate reader or discontinuously in SDS-PAGE gels. Models of the bound peptide substrates can be generated in silico to guide substrate selection and mutagenesis studies. CFP/YFP substrates have also been utilized to identify protease inhibitors. These in vitro and in silico methods can be used in combination with cell-based assays to determine if the targeted host protein affects viral replication.