Association of soluble matrix protein of Newcastle disease virus with liposomes is independent of ionic conditions.

An immunoaffinity method was designed for purification of a soluble form of the matrix (M) protein of Newcastle disease virus. The resulting M protein sedimented in a sucrose gradient as a small complex. This purified M protein associated with liposomes containing a net neutral, negative, or positive charge. The liposomes were composed of phosphatidylcholine, cholesterol, and a third lipid which provided the charge. The M protein-liposome associations were not prevented by high salt conditions. These observations are consistent with a nonelectrostatic association between the M protein and liposomes. Monoclonal antibodies to three separate epitopes of the M protein were all able to bind M protein complexed with liposomes, suggesting that the three M protein epitopes are not directly involved in the interaction between the M protein and liposomes. The M protein was also able to associate with liposomes lacking cholesterol implying that cholesterol does not play a substantial role in the M protein-liposome interaction.