THE STRUCTURE OF TURNIP YELLOW MOSAIC VIRUS (TYMV) AT 3.2 RESOLUTION. M. A. Canady, S. B. Larson, J. Day, and A. McPherson, Department of Biochemistry, University of California, Riverside CA 92521 USA

The structure of turnip yellow mosaic virus has been solved to 3.2 using molecular replacement, multiple isomorphous replacement (MIR), and molecular averaging. Crystals were of space group P6₄22 with unit cell dimensions a=b=515.5, c=309.4 . Native and heavy atom data were collected at Brookhaven National Laboratory at the University of California, San Diego. Using cowpea chlorotic mottle virus as a model, phases were computed and the map was averaged. A difference Fourier synthesis using the phases from the averaged map and data collected from two platinum derivatives allowed us to determine the positions of the heavy atoms. A polyalanine model was built into an averaged MIR map. Correct sidechains were built into an averaged map phased by the refined polyalanine model. The structure was refined using conjugate gradient minimization, simulated annealing, and individual restrained B factor refinement to an R-value of 18.7% with a free R-value of 19.3% to 3.0 . The structure of the virion at high resolution resembles the predictions made at low resolution, with the pentameric and hexameric coat protein assemblies protruding quite prominently, forming deep valleys at the pseudo-threefold axes between the A, B, and C subunits. The quasi-equivalent A, B, and C subunits adopt the jellyroll fold and are very similar in structure. The N termini, which had been found on the outside of the virion immunogenically, are found in the interior of the virion, and the first 26 residues of the A subunit are disordered. The N termini of the B and C subunits, which are completely visible, interact at the interior of the pseudo 6-fold axes, forming annuli. The C termini are exterior to the virion. All three histidines present in the coat protein are found on the inside of the virion, and may confirm the prediction that these residues bind RNA.