Aggregation of VSV M protein is reversible and mediated by nucleation sites: implications for viral assembly

Purified M protein of VSV has been reported to aggregate at low NaCl concentration. Using light scattering, analytical centrifugation, and electron microscopy (EM), we have studied this phenomenon. Our results demonstrate that self aggregation of M protein can be reversed by increasing the salt concentration. Below 250 mM NaCl, there is an equilibrium between aggregates and monomeric M protein. Most importantly, we demonstrate that aggregation only occurs in the presence of nucleation sites and that these sites are sensitive to trypsin. We have found conditions under which these nucleation sites can be eliminated, after which M remains soluble even at low salt concentration. Finally, using EM, we show that the aggregates of purified M protein share common structural aspects with the previously described internal "cigar" around which the nucleocapsid is wrapped. These new results help to explain why M is a soluble protein in the cytoplasm of the infected cell just up to the moment that it is integrated into the budding virion.