44 R. MARKHAM 



III. The Protein Components of Plant Viruses 



As has already been indicated, the greater part, amounting in some cases 

 to over 90 %, of the weight of virus particles consists of protein. Up to a few 

 years ago proteins were known to have very remarkable properties, which 

 had been determined from the study of enzymes, hormones, blood pigments, 

 and other biologically active substances having a protein nature, and were 

 regarded as the quintessence of life itself. In fact, according to one eminent 

 authority, life is "the mode of existence of the proteins." Recently, of course, 

 the recognition of the importance of nucleic acid in living processes has 

 resulted in a decrease of the popularity of the proteins, and, in all probability, 

 the pendulum may have swung too much in the other direction. The very 

 fact that the viruses, which are among the smallest self-perpetuating systems 

 known, invariably are associated with protein, and that of a highly specific 

 type, suggests that protein plays an important part in their economy. 

 Fortunately the study of virus proteins has not been neglected, because it 

 holds out the chance, slim as it may seem, that, in the study of the two 

 major components of the viruses, one might possibly disentangle the mysteri- 

 ous but fundamental code relating the structure of nucleic acids to that of 

 the proteins. At the present moment, the prospects of a successful conclusion 

 to this work are not as happy as one might hope, because the functions of 

 the two major components of the plant viruses are by no means fully under- 

 stood, but there is little doubt that some important discoveries will result 

 from this type of research. 



What then is known about the structure of proteins in general, and of the 

 virus proteins in particular? All proteins consist in the main of a-amino 

 acids, compounds having the structure R — CHNHa — COOH, where R is a 

 residue, or side chain, and usually joined to the remainder of the amino acid 

 molecule in such a way as to make the a-carbon atom levorotatory. In the 

 simplest amino acid, glycine or a-aminoacetic acid, of course, R is a hydrogen 

 atom and then the possibility of optical isomerism does not exist. In the 

 other cases the R-groups are all characteristic, and in normal protems some 

 20 are to be found, although in certain proteins only 16 or 17 of the possible 

 amino acids may be present, while in others there may be amino acids of a 

 quite unique structure. It will be appreciated that the possible number of 

 R-groups is to all intents unlimited, and it is a point of interest that, by and 

 large, the known organisms can make do with only 20 of the amino acids for 

 making their proteins. 



A. Polypeptides 

 The amino acids are joined together as the amides of the adjacent carboxyl 

 groups in the chain, by what are called peptide links. Thus a polypeptide 

 chain has the general structure: 



