260 H. FRAENKEL-CONRAT AND K. NARITA [14 



Direct chromatography of a hydrolysate of the acidic peptide also indicated 

 the presence of acetic acid. It thus appears estabUshed, that the structure 

 of the acidic peptide is acetyl-ser-tyr. Thus acetylation may well be 

 responsible for the absence of A^-terminal groups in TMV. The above pep- 

 tide probably represents the acetyl- A^-terminal sequence of the virus protein, 

 although its occurrence as a branch chain is not excluded. This is believed 

 to be the first instance* where an acetyl group has been definitely shown 

 to be a component of a simple protein, and has been allocated to a specific 

 site in that protein. 



In conclusion, then, TMV protein seems to be composed of peptide chains 

 of a molecular weight of about 18,000 which instead of a free amino 

 end terminate in acetyl-ser-tyr- and which have a thr-gly-ser-pro-ala-thr 

 sequence at the carboxyl end. About six polypeptide chains are united into 

 a definite aggregate (A-protein) resembling native proteins in having masked 

 — SH groups and tyrosine-carboxylate bonds. The A-protein tends to aggre- 

 gate to particles resembhng in shape the original virus, but lacking its 

 stability. 



It appears probable that the — SH group (one per chain) is hydrogen- 

 bonded, playing the role of an acceptor, since its H atom is free. In the 

 degradation of the virus, about 5000 new acid groups appear and become 

 titrable which may be originally present as masked carboxyl groups, hydro- 

 gen bonded in their undissociated form. To these groups may be attributed 

 the remarkable stability and other surface properties which differentiate the 

 intact virus from the isolated protein. These same groups appear to exchange 

 their hydrogen for lead and with lesser affinity other divalent metals. 



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* See, however, article by J. I. Harris (p. 335). — Ed. 



