Geminiviruses must be targeted to the nucleus to initiate their replication as each cell becomes infected however, after its replication the viral DNA must be transported from the nucleus to the cell periphery to allow its translocation to an adjacent cell. We have shown that the coat protein (CP) is responsible for the nuclear targetting of the viral DNA, via a nuclear localisation signal, and that the movement protein (MP) is required for cell to cell movement of the virus, or nucleoprotein (CP-DNA) complex. It is important to characterise the complex interactions involved in these processes in order to have a greater understanding of the infection process. We have established a number of collaborative programmes to facilitate our studies. For example, a high resolution structural model of the MSV capsid is allowing us to predict, and confirm experimentally, regions of the CP involved in capsid subunit or DNA interactions and surface residues likely to be important for host or vector-specific interactions. Mutagenesis of the MP and CP have led to the identification of residues required for encapsidation and for systemic (or cell to cell) infection as well as for pathogenicity of MSV. Expression of the proteins in heterologous systems and host cells has allowed us to localise the proteins to specific cellular compartments and a transgenic approach has shown the importance of the MSV and bean yellow dwarf virus MPs in pathogenicity and their likely homologous functions in monocots and dicots.
 |
A diagramatic representation of the infection cycle of Maize
Streak Virus with particular reference to iner and intra-cellular
movement. CP/CP* - coat protein with (*) or without an active nuclear localization signal; MP - movement protein; Lu,Su,Ss,Ls - the virion sense spliced(s), unspliced(u), long(L) and short(S) transcripts (Wright et al., 1997) required for expression of the CP and MP ; ? - exact mechanism is not clear. |
|
The pseudo-atomic model fitted to the reconstruction density map of the maize streak virus particle. The structural model was produced based on homology of the MSV coat protein with that of satellite tobacco necrosis virus and on cryo-electronmicroscopy observations (Zhang et al., 2001). Currently Carolina is using mutagenesis, replication and insect transmission studies to confirm the fidelity of the model. |
 |
 |
The mastrevirus movement proteins (MPs) cause an infection-like phenotype when expressed in transgenic plants. Nicotiana tabacum plants expressing the bean yellow dwarf virus MP show symptoms typical of BeYDV infection (left). Expression of the maize streak virus MP in N. benthamiana also results in geminivirus-like symptoms. |
|
(right) The leaf at the top is from a plant that does not express the MP. These results suggest that the MPs are major pathogenicity determinants and that the BeYDV and MSV MPs are functionally homologous |
 |