Capturing enveloped viruses on affinity grids for downstream cryo-electron microscopy applications.

Electron microscopy (EM), cryo-electron microscopy (cryo-EM), and cryo-electron tomography (cryo-ET) are essential techniques used for characterizing basic virus morphology and determining the three-dimensional structure of viruses. Enveloped viruses, which contain an outer lipoprotein coat, constitute the largest group of pathogenic viruses to humans. The purification of enveloped viruses from cell culture presents certain challenges. Specifically, the inclusion of host-membrane derived vesicles, the complete destruction of the viruses, and the disruption of the internal architecture of individual virus particles. Here, we present a strategy for capturing enveloped viruses on affinity grids for use in both conventional EM and cryo-EM/ET applications. We examined the utility of affinity grids for the selective capture of human immunodeficiency virus (HIV) virus-like particles (VLPs), influenza A, and measles virus (MeV). We applied Nickel-nitrilotriacetic acid (Ni-NTA) lipid layers in combination with molecular adaptors to selectively adhere the viruses to the affinity grid surface. This further development of the affinity grid method may prove essential for the gentle and selective purification of enveloped viruses directly onto EM grids for ultrastructural analysis.