Characteristics of virus-cell complexes

Abstract

T HE interaction of a virus particle with a cell can be subdivided into a series of steps. At first the virus particle establishes contact with the cell (attachment), then the nucleic acid of the particle becomes released from its coats and reaches the cellular site where its replication is going to take place (penetration). The nucleic acid then replicates (replication), and the protein units which constitute the viral envelopes are synthesized. The nucleic acid and these coat units become assembled together into virus particles (maturation). Finally, the progeny virus particles are freed from the cell in which they were produced (release). Not all these steps must necessarily take place in all cases. For instance, naked nucleic acid can penetrate the cell, thus eliminating attachment and part of the penetration process; or release may be absent. In some cases the viral nucleic acid reaches a cell from the parent cells; in other cases the multiplication of the nucleic acid and the sy-nthesis of coat material may not take place. Every virus, however, must be able to cause the complete series of steps, since this is the basis for the definition of the virus: i.e., a particle which can infect a cell and can multiply in a cell, giving a product which is identical with the infecting particle itself. The fact that in specific cases the interaction of a virus with a cell may not involve the complete series of steps delineated shows that the type of interaction between the virus and the cell may be different. The consequences of the presence of the virus for the cell vary greatly, according to the type of interaction: in some cases, the cell is destroyed, in some it may be essentially unchanged, in others it may be changed into a cancer cell. Thus, also, the consequences of the infection of an organism depend on the type of virus-cell interactions which become established between the virus and the cells of the body. Knowledge of virus-cell interactions is, therefore, fundamental to understanding the biology and the pathology of virus infection. The step which most markedly distinguishes different types of virus-cell interaction is the replication of the viral nucleic acid. In order to appreciate this, the following points are worth considering. The viral nucleic acid must be considered to be endowed with two fundamental properties. One is that of a source of information for protein synthesis. In the case of DNA viruses, the genes of the virus are reproduced in the form of messenger RNA which, upon association with ribosomes, then causes the synthesis of polypeptide chains of specific sequences. In the case of RNA viruses, the viral RNA itself functions as messenger. Thus, in both cases viral proteins are produced in the cells, and the structure of these proteins is determined by the sequence of nucleotides in the viral genes. These proteins in turn have specific functions in the infected cells. Some of them constitute the envelope of the progeny virus particles. Others alter the cellular functions in a wa) which is suitable for the reproduction of the virus. Thus, a virus-infected cell becomes different from a similar but uninfected cell, because of the presence of these viral proteins. The other fundamental property of the viral nucleic acid is that of reproduction. The results of this reproduction are molecules of nucleic acid endowed with the characteristics of the infecting molecules. Reproduction, however, is not a simple act; it follows a complex path and has complex requirements. The complexity of the process is indicated by the fact that the nucleic acid must change its conformation in order to replicate. For instance, double-stranded molecules must separate into single strands, a process which can take place at the moment reproduction takes place. Singlestranded molecules must build their doublestranded counterparts. The detailed mechanisms involved in all these cases are still obscure for the most part. The complexity of the requirements derives from the multiplicity and