Virus coat proteins, structure-function

Antiparallel beta (P) structures comprise the second large group of protein domain structures. Functionally, this group is the most diverse it includes enzymes, transport proteins, antibodies, cell surface proteins, and virus coat proteins. The cores of these domains are built up by p strands that can vary in number from four or five to over ten. The P strands are arranged in a predominantly antiparallel fashion and usually in such a way that they form two P sheets that are joined together and packed against each other. 

It might thus be expected that this structure has been incorporated in many other proteins wherever there was a need for its characteristic function. A requirement of binding nucleotides to proteins occurs in energy transfer systems and in the molecular reproduction of nucleic acids. Consequently, it is possible that this molecular fossil may be found in such diverse proteins as tRNA synthetase, ribosomal proteins, and virus coat proteins. The recognition of its structure by sequence homology or from X-ray structure determinations may also give guidance as to function where none is known. 

Not all viruses have a protein coat or envelope. However, a protein structure called the capsid is present, and it becomes the only protection for the nucleic acid core in the absence of an envelope. Envelope proteins, when present, are frequently glycosylated. The most important virus-specific proteins are the transcriptases. In studying viral replication pathways by utilizing various inhibitors as molecular probes, some processes unique to viruses have been discovered. This may allow the designing of specific replicative inhibitors that might function clinically at acceptable toxicity levels. For example, RNA viruses either... 

Viruses very small, infective particles, which are not retained by ultrafine filters that hold back the smallest bacteria. They consist of RNA or DNA enclosed in a protein coat, and they can replicate (multiply) only within a suitable live host cell. They suppress the genetic information of the host cell, and exploit the ribosomes, energy-producing mechanisms and various enzymes of the host in support of their own replication. There is a wide variety of different virus types, differing in the type of nucleic acid (RNA or DNA, sin e-stranded or double-stranded, linear or circular), structure of the protein coat, mode of infection, and mechanism of replication. Virus infections often cause functional disturbances of the host organism, known as virus diseases or viro-ses. 

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