AGGREGATION OF VSV M-PROTEIN IS REVERSIBLE AND MEDIATED BY NUCLEATIONSITES - IMPLICATIONS FOR VIRAL ASSEMBLY

Purified M protein of VSV has been reported to aggregate at low NaCl c oncentration. Using light scattering, analytical centrifugation, and e lectron microscopy (EM), we have studied this phenomenon. Our results demonstrate that self aggregation of M protein can be reversed by incr easing the salt concentration. Below 250 mM NaCl, there is an equilibr ium between aggregates and monomeric M protein. Most importantly, we d emonstrate that aggregation only occurs in the presence of nucleation sites and that these sites are sensitive to trypsin. We have found con ditions under which these nucleation sites can be eliminated, after wh ich M remains soluble even at low salt concentration. Finally, using E M, we show that the aggregates of purified M protein share common stru ctural 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. (C) 19 95 Academic Press, Inc.