Optimized method for isolation of immature intracytoplasmic retroviral particles from mammalian cells

To biochemically and structurally characterize viral intracytoplasmic particles (ICAPs), a sample of high purity and homogeneity is usually required. Production of ICAPs in the system closely related to their natural host cells is crucial for the analysis of host-cell binding proteins involved in ICAPs assembly, transport and budding. However, this approach is often hampered by problems with low yield of the ICAPs due to either low expression or fast release from the host cell. Another obstacle may be a low stability or fragility of the intracellular particles. The published methods for ICAPs isolation often involved several time-consuming centrifugation steps yielding damaged particles. Other papers describe the ICAPs production in non-natural host cells. Here, we optimized the method for purification of unstable Mason-Pfizer monkey virus (M-PMV) ICAPs from non-human primate derived cells, commonly used to study MPMV replication i.e. African green monkey kidney fibroblast cell line (COS-1). Our simple and rapid procedure involved separation of the intracytoplasmic particles from the cell debris and organelles by differential, low-speed centrifugation, their purification using sucrose velocity gradient and final concentrating by low-speed centrifugation. Importantly, the method was established for unstable and fragile M-PMV intracytoplasmic particles. Therefore, it may be suitable for isolation of ICAPs of other viruses.