An isolated pore-forming polypeptide is disclosed which comprises a naturally-occurring plugging module and a naturally-occurring pore domain, wherein at least one amino acid of the pore-forming polypeptide is mutated to generate a protease cleavage site, serving to at least partially remove the plugging module from the pore domain. The pore forming polypeptides may be inserted into an encapsulating particle and positioned such that it is capable of forming a pore through the lipid layer of the particle in a presence of the protease.