The open reading frame III product of cauliflower mosaic virus forms a tetramer through a N-terminal coiled-coil

The open reading frame III product of cauliflower mosaic virus is a protein of 15 kDa (p15) that is essential for the virus life cycle. It was shown that the 34 N-terminal amino acids are sufficient to support protein-protein interaction with the full-length p15 in the yeast two-hybrid system. A corresponding peptide was synthesized and a recombinant p15 was expressed in Escherichia coli and purified. Circular dichroism spectroscopy showed that the peptide and the full-length protein can assume an alpha-helical conformation. Analytical centrifugation allowed to determine that p15 assembles as a rod-shaped tetramer. Oxidative cross-linking of N-terminal cysteines of the peptide generated specific covalent oligomers, indicating that the N terminus of p15 is a coiled-coil that assembles as a parallel tetramer. Mutation of Lys22 into Asp destabilized the tetramer and put forward the presence of a salt bridge between Lys22 and Asp24 in a model building of the stalk. These results suggest a model in which the stalk segment of p15 is located at its N terminus, followed by a hinge that provides the space for presenting the C terminus for interactions with nucleic acids and/or proteins.