SCAFFOLDING PROTEIN GPD
Are they alive or are they not alive? Strictly speaking, viruses are not alive, because they cannot reproduce by themselves. Very often they are associated with causing harm to living organisms and being the source of diseases, like for instance influenza virus, cold virus or HIV. Sometimes, surprisingly, from a human point of view, they can be good viruses, infecting bacteria that might otherwise be harmful. Viruses need another organism to reproduce. Some viruses use bacteria as their host and such viruses are called bacteriophages, a word of Greek origin meaning bacterium eater. The genetic information of a virus can be single-stranded or double- stranded DNA (deoxyribonucleic acid) or RNA (ribonucleic acid). This viral genetic material is generally stored inside a container called the viral capsid, sometimes surrounded by an additional lipid membrane layer as for instance is the case with HIV. Some viruses, like the T4 bacteriophage, have long channels attached to the capsid, with additional spikes to attach to cells, to penetrate cellular membranes and inject the genetic material into the host cell, working more or less like a needle. The capsid is often regularly shaped and is made of proteins. Viral capsid contains many protein units of the same kind.
In the case of bacteriophage phiX174 there are 240 copies of one kind of subunit 60 each of three other kinds and 12 of a fifth kind. The structure here shows an example of one of the 240 scaffold proteins. From a study of such structures one can draw conclusions, how the virus builds its capsid shell. Bacteriophage phiX174 first builds a preliminary version of the capsid and then integrates the viral genetic material. The early stage of this construction, the so-called procapsid, contains this scaffolding protein. Its main purpose is to help assemble the whole virus. Once virus assembly is nearly complete, this scaffold protein is removed from the capsid and the infectious viral particle results as the final step of this construction. The main function of this protein is to prevent incorrect assembly of the capsid.
Protein Data Bank (PDB)
Dokland, T. Bernal, R.A. Burch, A. Pletnev, S. Fane, B.A. Rossmann, M.G.; "The role of scaffolding proteins in the assembly of the small, single-stranded DNA virus phiX174."; J. Mol. Biol.; (1999) 288:595-608 PubMed:10329166.
author: Arno Paehler