Virus-like particles (VLPs) are self-assembling multi-protein nanoparticles with similar structural organization and conformation as viruses but without viral genome. The size of the VLP is about 100-150nm. It is secreted from the surface of the cells that express target membrane proteins (MPs). The purified VLPs have the target MPs inserted in a complete bilayer phospholipid membrane structure, mimic the natural membrane-penetrating state of the protein.