Abstract
The calcium ions normally present in the protein shell of the satellite tobacco necrosis virus (STNV) have been removed by EDTA. The radius of the EDTA-treated virion was found to be dependent upon pH. The radial increase was 2% at pH 5.0,4.5% at pH 6.5, and 7% at pH 7.0 as determined by analytical ultracentrifugation and X-ray crystallography. The virion could be recontracted by lowering the pH or by the addition of divalent cations, calcium ions being the most efficient. At pH 7.0 and above, the EDTA-treated virion was sensitive to ribonucleases and to trypsin. The cleavage site for trypsin is at Arg 28, which is near the N terminus of the shell domain of the subunit, but at a point which is well buried in the native structure. Crystals of the expanded STNV particles at pH 5.0 and 6.5 have been produced.
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