Date of Award


Degree Type


Degree Name

Doctor of Philosophy


The potexvirus family consists of flexuous helical plant viruses that are architecturally similar (Richardson et al, 1981). The behaviour of the coat proteins of several of these viruses (barrel cactus virus (BCV), foxtail mosaic virus (FTV), viola mottle virus (VMV), potato virus X (PVX) and tulip virus X (TVX)) is compared to determine whether related viruses assemble by a common pathway.;The assembly of the proteins was examined in the absence of the RNA, under various conditions of pH, temperature, ionic strength and protein concentration. The structure of the products was determined using optical diffraction.;The formation of small aggregates of the proteins examined here appears to involve the formation of a hexamer/heptamer, in most cases from a trimer. Disks, like those formed by the proteins of other potexviruses (papaya mosaic virus (PMV) and clover yellow mosaic virus (CYMV)) and unrelated helical viruses, were not observed. The formation of trimers and hexamers may be common to all potexviruses, but disk formation may not be a requirement for reconstitution. The potexviruses do not appear to share a common sub-assembly pathway, nor can such a pathway be predicted from the structure of the final product.;BCV and FTV proteins form virus-like polymers over a wide pH range, indicating that protein-protein interactions are important for the assembly of these viruses, as well as for PMV and CYMV. The tubular polymers of VMV, PVX and TVX proteins are not virus-like. VMV and PVX proteins form stacked rings. VMV and TVX proteins form multiple helices. Therefore, protein-RNA interactions are more important than protein-protein interactions in directing the assembly of their capsids.;Carboxyl-carboxylate interactions, like those that regulate the assembly of tobacco mosaic virus protein and possibly that of PMV protein, do not seem to be necessary for all potexviruses. BCV and FTV proteins both polymerize extensively at high pH where carboxyl-carboxylate interactions would be unstable. Thus the nature of protein-protein interactions controlling capsid assembly is not the same for all helical viruses or even for all potexviruses.



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