On the Role of Protein 3CD in Formation of the Poliovirus Replication Organelle

Picornaviruses hijack host membranes to create an organelle that serves as the site of viral genome replication. This virus-induced organelle can be observed by EM at a time when viral RNA is accumulating exponentially and infectious virus can be isolated. Proteins encoded by the P2-region of the genome have been thought to be necessary and sufficient for transformation of intracellular membranes into a compartment for viral replication. However, there is increasing evidence that proteins encoded by the P3-region serve important roles in formation of the replication organelle. We have made the unexpected observation that the poliovirus (PV) protease and RNA-binding protein, 3CD, contributes to formation of the replication organelle and is needed for this organelle to efficiently transfer replicated genomes from the site of synthesis into capsids. The role of 3CD in formation of the replication organelle is concentration dependent and can be complemented in trans, suggesting an interaction of 3CD with a host factor. Recently, the Altan-Bonnet lab showed that phosphatidylinositol-4-phosphate (PI4P) increases in abundance during picornavirus infection and localizes to the replication organelle. How a lipid contributes to formation and/or function of the replication organelle remains to be elucidated. We have now shown that ectopic expression of 3CD is sufficient to alter the localization and increase the abundance of PI4P lipids in cells. In addition, our biochemical, biophysical and computational experiments support the conclusion that PV 3CD is a phosphoinositide-binding protein. We are currently pursuing the hypothesis that 3CD is recruited to virus-induced membranes containing PI4P and that the level of 3CD and 3CD-3CD interactions leads to formation of the replication organelle and the large vesicular clusters observed in cells infected by PV. We suggest these large vesicular clusters may be required for virus assembly.