482 A. TSUfilTA WD H. I- R AKX KKI,-r( )N HAT 



protein is also greatly dependent on ionic strength, j)ll, and tempciature 

 (Schranun and Zillig, IBoo; Kramer and Wittniann, 1958 j. 



The diflieultii's in obtaining i)liysicocheniical parameters for the TMV 

 protein niononici' are due to its marked tendency to aggregate, thus 

 expressing its biological function. With increasing ionic strength and 

 temperature, and witli a i)H optinnun of about 5, the protein aggregates 

 to discs, rods, and finally to rods of ever-increasing lengths, indistin- 

 guishable by all criteria, exce])t that of irregular and, under cei-tain con- 

 ditions, excessive length, from TMV jjarticles. These observations prove 

 that the general architecture of the virus particle is a function only of 

 the shape of the protein monomer, wliile the length of the particle, as 

 we shall see later, is determined by the length of the polynucleotide 

 chain. 



If the shape of the protein subunit determines that of the virus, the 

 cjuestion arises: what determines the shape of the protein? Recent 

 studies with ribonuclease and other proteins have yielded evidence that 

 the native conformation of a peptide chain may be a consequence of its 

 amino acid sequence alone (White and Anfinsen, 1959). A similar conclu- 

 sion appears justified in regard to the 3-dimensional structure of the 

 TMV protein chain, on the basis of the report by Anderer (1959) that 

 the denatured and structureless protein can readily be renatured by 

 dialysis from 6 M urea or 0.1 A" NaOH solution. These observations have 

 been confimied and extended in our laboratory. It thus appears probable 

 that the shape of the protein monomer represents the thermodynamically 

 most stable conformation of its peptide chain. It has not yet been 

 demonstrated in detail what bonds account for this structure, but the 

 occurrence of several chain segments folded into «-helices, interspersed 

 with non-helical areas and bends due to proline, or an accumulation of 

 those side chains which interfere with stable helix formation appears 

 probable (Blout et al., 1960). Quite possibly the folds are stabilized by 

 hydrophobic interactions of groups of ali])hatic side chains, as well as 

 by ionically linked and hydrogen-bonded polar residues. It must be 

 noted that 12 of the 16 serines occur within 15 residues from both ends 

 of the chain which are, in turn, quite low in basic and acidic residues; 

 further, the majority of the serines, leucines, and isoleucines, as well as 

 of the primary amide groups, occur in clusters along the chain. 



Certain specific bonds have been suggested as ])articipating in either 

 or both, chain conformation and interchain bonding. Thus, the — SH 

 group can be detected and titrated only in the presence of dcnaturating 

 reagents, and it is particularly unusual in its behavior in the intact 

 virus. When the virus is treated with iodine the — SH group becomes 

 substituted. The resultant sulfenyl iodide ( — -SI), usually quite a labile 



