Viral membrane-penetration protein, usually associated with the viral capsid or released through partial programmed disassembly of the capsid, that locally permeabilizes the bilayer of a host organellar membrane to allow viral escape and penetration into the cytoplasm. Viral membrane-penetration proteins often need to be activated, mostly through endosomal acidic pH or receptor binding to display their membrane penetrating activity. Non-enveloped viruses such as parvovirus, human reovirus, BDV, BTV, rotavirus, papillomavirus, Flock house virus permeabilize the host endosomal membrane to penetrate the host cytoplasm.