PROTEIN OF TOBACCO MOSAIC VIRUS
Authors: KLECZKOWSKI A. 
Published: January 2008

A typical infective rod‐shaped particle of tobacco mosaic virus (TMV) is composed of ribonucleic acid, of molecular weight about 2,000,000 and about 2200 helically arranged protein sub‐units, each with a molecular weight of about 18,000. The sub‐unit is made of a single chain of 158 amino‐acid residues of known sequence. The thread of the nucleic acid is sandwiched between protein sub‐units and follows their helical array about the long axis of the particle.

The virus is easily degraded, and the protein separated from the nucleic acid, by alkali. The protein thus obtained, called A‐protein, has particles of molecular weight about 100,000, composed of 6 protein sub‐units. When the pH is lowered to below 6, particles of the A‐protein polymerize. This is comparable to crystallization, with protein sub‐units fitting into a regular pattern as in the original virus particle.

Infectivity is a function of the virus nucleic acid and the nucleic acid free from protein is infective. The only known biological function of the protein is to stabilize the nucleic acid and protect it from ribonuclease and some other inactivating agents. A remarkable property of the A‐protein and of free virus‐nucleic acid is their ability to recombine to form ‘reconstituted’ virus, which is similar to the original virus.

Sap from plants infected with TMV contains not only the virus, but also free protein, called X‐protein, which is similar to and may be identical with the A‐protein. It also combines with virus‐nucleic acid to form virus‐like infective particles. X‐protein may be surplus virus protein not incorporated into virus particles, but some may come from virus particles, which readily shed a small proportion of their protein.

Walls of protein sub‐units seem to differ in various physico‐chemical aspects, such as electrokinetic potential at the surface and susceptibility to proteolytic enzymes as trypsin or papain. Polymerization, which conceals some of these walls, alters electro‐phoretic mobility and isoelectric point, and confers resistance to proteolytic enzymes.

Serological behaviour of TMV is entirely a function of its protein. Whether the protein sub‐units are all antigenically identical is uncertain. The fact that A‐protein can form several precipitation bands with antiserum to TMV in the gel‐diffusion precipitin test suggests that there are several antigenically different kinds of protein sub‐unit, but is not conclusive, for a homogeneous antigen can sometimes form several bands. Other facts, however, point to the conclusion that there are at least two, and possibly more, antigenically different kinds of protein sub‐units. If all the sub‐units have the same amino‐acid composition and sequence, but differ antigenically, this presumably means that the polypeptide chains may be folded differently.

A‐protein (and X‐protein) may have an antigenic determinant that becomes concealed when the protein is polymerized.

Some pronounced differences in serological behaviour between TMV and polymerized A‐protein (or X‐protein), on the one hand, and non‐polymerized A‐protein (or X‐protein), on the other, arise merely from differences in the size and shape of their particles. The differences in behaviour concern rate of precipitation with antisera, type of precipitate, ratio of weight of antigen to weight of antibody in precipitate and range of dilutions of antigens and of antisera at which precipitation occurs.

TMV is a very convenient material for work on molecular genetics, and results of work on amino acid replacements in the protein of naturally occurring and in chemically evoked mutants of the virus have already contributed considerably to the progress in this new but fast developing subject.

Only a part of the polynucleotide chain of the virus nucleic acid seems to be concerned with the synthesis of the virus protein. It is possible that the virus nucleic acid directs production of several proteins, some of which affect the host plant by their enzymic activity, and only one is incorporated into the virus particle.

Nucleic acid of TMV can stimulate protein synthesis in a cell‐free system made from E. coli containing ribosomes. A part of the synthesized protein resembles that of TMV.