In situ spatial organization of potato virus A coat protein subunits as assessed by tritium bombardment

Potato virus A (PVA) particles were bombarded with thermally activated trit ium atoms, and the intramolecular distribution of the label in the amino ac ids of the coat protein was determined to assess their in situ steric acces sibility. This method revealed that the N-terminal 15 amino acids of the PV A coat protein and a region comprising amino acids 27 to 50 are the most ac cessible at the particle surface to labeling with tritium atoms. A model of the spatial arrangement of the PVA coat protein polypeptide chain within t he virus particle was derived from the experimental data obtained by tritiu m bombardment combined with predictions of secondary-structure elements and the principles of packing a-helices and 13-structures in proteins. The mod el predicts three regions of tertiary structure: (i) the surface-exposed N- terminal region, comprising an unstructured N terminus of 8 amino acids and two 13-strands, (ii) a C-terminal region including two a-helices, as well as three 13-strands that form a two-layer structure called an abCd unit, an d (iii) a central region comprising a bundle of four a-helices in a fold si milar to that found in tobacco mosaic virus coat protein. This is the first model of the three-dimensional structure of a potyvirus coat protein.