The identification of this protein function provides a suitable target to prevent future disease outbreaks.
Exosomes are essential for virus exocytosis and transmission. The exportin family of proteins are responsible for transporting proteins from the nucleus to the cytoplasm in eukaryotes and are commonly found in exosomes. However, it is not known what exportins in exosomes do.
Recently, a group of researchers led by Professor Cui Feng from the Institute of Zoology of the Chinese Academy of Sciences have discovered a saliva protein, exportin 6, which serves as a means for transporting plant virions into exosomes, thus overcoming the barrier of insect salivary glands.
Plant viruses are major pathogens that have serious negative impacts on the global economy. The majority of plant viruses rely on specialised sap-feeding insects to spread.
One of the most destructive rice diseases in East Asian nations is caused by the Rice Stripe Virus (RSV) that is transmitted by the small brown planthopper. The last layer of protection against viral transfer from insects to plants is the insect salivary glands. How viruses get past the salivary gland barrier has thus far remained a mystery.
To determine the function of exportin proteins in exosomes, the researchers extracted exosomes from the saliva of about 30,000 planthoppers less than four mm in size. They discovered RSV virions in the saliva exosomes and found that these exosome-borne virions couple reproduce and wreak disease in rice plants.
The researchers reported that exportin 6 is necessary for the entry of RSV into exosomes. It seems like exportin 6 acts as a vehicle for transporting virions into exosome precursors via interactions with the cargo-sorting protein VPS37a. When the expression of exportin 6 gene was knocked down, the amount of RSV secreted in saliva and rice plants were reduced by 60 and 74 per cent respectively.
The study provides direct evidence that exportin 6 facilitates viral transmission from insect vectors to host plants by assisting viruses in overcoming salivary gland barriers. These findings suggest that exportin 6 would be a viable target for disrupting the viral transmission cycle and control future disease outbreaks.
Source: Lu et al. (2022). Key role of exportin 6 in exosome-mediated viral transmission from insect vectors to plants. Proceedings of the National Academy of Sciences, 119(36), e2207848119.