SITE-DIRECTED MUTAGENESIS CONFIRMS THE INVOLVEMENT OF CARBOXYLATE GROUPS IN THE DISASSEMBLY OF TOBACCO MOSAIC-VIRUS

Electrostatic repulsion between carboxylate groups across subunit inte rlaces has for many years been recognized as important in the disassem bly of simple plant viruses. In the coat protein of tobacco mosaic vir us (TMV), the amino acids Glu50 and Asp77 have been proposed as exampl es of such carboxylate groups. Site-directed mutagenesis has been used to replace these amino acids by Gin and Asn, respectively. Increased virion stability, together with reduced infectivity and reduced capaci ty for long-distance transport within the host plant confirms that the negative charges on the side chains of these amino acids are involved in the disassembly of TMV. Mixing purified mutant coat proteins with wild-type virions under appropriate conditions stabilizes the virions to alkaline disassembly and reduces their infectivity. It is suggested that transgenic plants expressing such mutant coat proteins could hav e enhanced resistance to virus infection. (C) 1995 Academic Press, Inc .