The Primary Structure of the Coat Protein of Alfalfa Mosaic Virus Strain VRU. A Hypothesis on the Occurrence of Two Conformations in the Assembly of the Protein Shell

Abstract

The complete primary structure of the coat protein of strain VRU of alfalfa mosaic virus (AMV) is reported. The strain is morphologically different from all other AMV strains as it contains large amounts of unusually long virus particles. This is caused by structural differences in the coat protein chain. The amino acid sequence was mainly established by the characterization of peptides obtained after cleavage with cyanogen bromide and digestion with trypsin, chymotrypsin, thermolysin or Staphylococcus aureus protease. The major sequencing technique used was the dansyl-Edman procedure. The VRU coat protein consists of 219 amino acid residues corresponding to a MW of 24,056. Compared to the coat protein of strain 425, 15 amino acid substitutions were localized. Most of them have a conservative character and may be explained by single-point mutations. A correction is given for the AMV 425 coat protein: Asn-216 was shown to be Asp-216. The prediction of the secondary structure for the 2 viral coat proteins was not significantly influenced by the various amino acid substitutions except for the region containing residues 65-100. The AMV coat protein may occur in 2 different conformations favoring its incorporation into either a pentagonal or hexagonal quasi-equivalent position in the lattice of the protein shell. The substitutions in the abovementioned region of the VRU coat protein probably caused a strong preference for the hexagonal lattice conformation. The model is supported by preliminary sequence data of the same coat protein region in AMV 15/64, a strain morphologically intermediate between 425 and VRU.