A Precipitation-Based Assay to Analyze Interactions of Viral Particles with Cytosolic Host Factors

: Since viruses are obligate intracellular parasites, viral particles, subviral structures, and viral proteins enlist the support of host proteins to foster intracellular transport, viral gene expression, replication, and evasion from antiviral host responses. We have devised a biochemical in vitro method to analyze specific interactions of cytosolic factors with capsids of herpes simplex virus and to characterize host proteins that specifically coprecipitate with different types of viral particles by immunoblotting, mass spectrometry, and immunoelectron microscopy. Our method bridges the gap between assays such as co-immunoprecipitation and yeast-two-hybrid approaches that determine direct binding between individual subunits of protein complexes and microscopy methods that analyze the dynamic interplay between intact viral particles and host factor complexes in intact cells. Our protocol can be extended to functional analyses of herpesvirus capsids and other viral structures with more complex host structures such as microtubule transport, genome uncoating at nuclear pores, or capsid envelopment at host membranes.