Movement proteins

Successful infection of a plant by a plant virus depends on its ability to move systemically from the cell initially infected to neighbouring cells in order to spread infection. Unlike viruses that infect animal cells no cellular receptors are known to be involved in plant cell infection. Additionally, as plants have robust cell walls viruses cannot penetrate through them unaided. Movement proteins are non-structural proteins encoded by many, if not all, plant viruses to enable their movement from one infected cell to neighbouring cells. Some plant viruses express more than one movement protein. The movement protein of Tobacco mosaic virus (TMV) has been most extensively studied. Plant viruses can also be transported over longer distances through the host plant in the vascular system via the phloem.

Plant virus movement between cells
The most common mechanism by which plant viruses move between plant cells via plasmodesmata. Plasmodesmata are pores between plant cell walls that allow the plant cells to communicate with each other. Plasmodesmata usually only allow the passage of small diffusible molecules, such as various metabolites. Neither virus particles nor viral genomic nucleic acid can pass through plasmodesmata unaided.

Function of movement proteins
Movement proteins function to modify the plasmodesmata by one of two well understood molecular mechanisms. The movement proteins of many plant viruses form a transport tubule within the pore of the plasmodesmata that allow the transport of mature virus particles. Examples of viruses that use this mechanism are Cowpea mosaic virus (CPMV) and Tomato spotted wilt virus (TSWV). The second mechanism by which movement proteins aid the viruses infection of cells is by the movement protein associating with and coating the genome of the virus to cause the ribonucleoprotein complexes to be transported through plasmodesmata into neighbouring cells. An example of a plant virus whose movement protein acts via this mechanism is TMV. The movement protein of TMV is 30Kda and has also been suggested to have other roles during infection.