Proton-magnetic-resonance studies on the coat protein of alfalfa mosaic virus.

Abstract

A model is postulated for the alfalfa mosaic virus (AlMV) coat protein (Mr 24250), consisting of a rigid core and a flexible N‐terminal part of approximately 36 amino acid residues. This model is based on a proton nuclear magnetic resonance study of the AlMV coat protein. The NMR spectrum of the coat protein shows several motionally narrowed resonances which strongly decrease in intensity when 25 N‐terminal amino acid residues are removed by a mild tryptic treatment. The motionally narrowed resonances, therefore, are assigned to the N‐terminal region. Specific assignments in the spectrum for tyrosine‐21, threonine‐15 and the N‐terminal N‐acetyl group can be made. The model is supported by photochemically induced dynamic nuclear polarization experiments on the intact protein and the trypsinated protein, which indicate that tyrosine‐21 is the only tyrosine which is easily accessible from the solvent. Furthermore, NMR spectra were obtained from a polymerisation product of the coat protein, the so‐called 12‐S product in which only the sharp resonances of the N‐terminal part are observed. Apparently, due to the large particle mass (about 400 kDa), the resonances of the core part of the protein are broadened to such an extent that they are no longer observable. The part of the spectrum which originates from the mobile region of the coat protein can be simulated by calculating a spectrum for a random coil peptide of about 36 N‐terminal amino acids. This indicates that the flexible region extends to about 36 amino acid residues from the N terminus and that it probably occurs in a random coil structure. Evidence is presented for a change in structure of the protein in solution when it assembles into the virus particle. This conclusion is based on an observed spectral change of histidine‐220.