Scaffold protein named shock12/28/2023 ![]() ![]() Most dsDNA tailed phages assemble their capsid from multiple copies of one capsid protein. These differences would affect receptor attachment and uncoating in viruses that otherwise use any of the five-fold vertices during infection (e.g., picornaviruses) perhaps explaining the rare occurrence of prolate heads among eukaryotic viruses. In a prolate head there are two types of five-fold vertices, the ten surrounding the cylindrical section and the two five-folds at the center of the caps. As was pointed out by Moody, elongation of the head in one direction does not affect receptor binding sites in bacteriophages, because only the vertex where the tail is attached to the head is functional for cell binding. Prolate heads occur in several genera of plant, fungal and animal viruses, but are more common among bacteriophages, where they are observed in about 15% of studied viruses. Such a structure is described by two triangulation numbers: a T number is used for the terminal caps and a Q number for the cylindrical section. A different way of increasing capsid size is assembling an elongated or prolate head, by inserting an additional cylindrical section between the two icosahedral end-caps. This implies that virus assembly must ensure that the capsid proteins form a unique structure with a specific T number. However, for each virus there is usually a dominant capsid organization. Not all T values are permissible (although some disallowed T numbers can occur in practice ) as some T numbers cannot be arranged with quasi-symmetric environments. A T = 1 particle consists of twelve pentameric capsomers, whereas a capsid with a higher T number contains both pentameric and hexameric capsomers. For example, if there are T (the triangulation number) quasi-equivalent positions, there are T × 60 capsid proteins in a capsid. A bigger capsid can be made by placing several monomers into the icosahedral asymmetric unit with “quasi-equivalent” environments, making the total number of capsid proteins a multiple of 60. A minimum of 60 copies of the capsid protein is necessary to make an icosahedral particle, in which case all the subunits have identical environments. Multiple copies of viral capsid proteins are frequently assembled into icosahedral shells, as was predicted by Crick and Watson and later confirmed by Caspar. Many bacterial and eukaryotic viral capsids have similar organization. Thus, for instance, we make no mention of Corticoviridae and Plasmaviridae families of phages. In this review we discuss only those groups of phages for which a significant amount of knowledge on assembly has been accumulated. Nevertheless, even within the same group of phages, there are notable differences in the assembly pathway, size and symmetry of the capsid and tail, positions of individual genes in the genome, the number of structural proteins and whether or not they are cleaved during assembly. Moreover, the bacterial injectosome, the hook of flagella and phage tails all use molecular ruler proteins in order to assemble a complex of correct length. Additionally, the capsid protein fold common to dsDNA tailed phages also occurs in herpesviruses, as well as in bacterial molecular compartments. The majority of viruses undergo proteolytic cleavages during assembly that are often essential to trigger the next assembly step. For example, most bacteriophages as well as herpesviruses, adenoviruses, poxviruses and the giant mimivirus make an empty protein shell that is subsequently packaged with the viral genome. There are substantial similarities in the assembly processes of all types of viruses, and of various cellular complexes. ![]()
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