Virion-associated viral fibroblast growth factor stimulates cell motility

Abstract

The Autographa californica M nucleopolyhedrovirus (AcMNPV) viral fibroblast growth factor (vFGF) has functional parallels to cellular FGFs. Deletion of the AcMNPV vfgf has no obvious phenotype in cell culture but delays the time of insect death. Here, we determined vFGF production during virus infection. vFGF was detected at 24 hours post infection and through the reminder of the infection cycle. Since vFGF is thought to be a secreted membrane-binding protein and virions acquire an envelope derived from the cell membrane, we examined virions for the presence of vFGF using microscopy, flow cytometry, and affinity chromatography. We found that vFGF associated with virions. Furthermore, budded virus carrying vFGF had more affinity to heparin than vFGF-deficient budded virus, consistent with the affinity of FGFs for heparan sulfate proteoglycans. Although the function of virion-associated vFGF is not clear, we found that virion-associated vFGF stimulated cell motility and affected virus attachment.

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“…Accumulation of AcFGF is not observed until 24 h p.i. in TN-368 cells (Lehiy et al, 2009), but this is presumably due to the low levels of synthesis characteristic of signaling proteins (Detvisitsakun et al, 2005). …”

“…vfgfs contain predicted N-terminal hydrophobic signal sequences, and they are secreted products (Detvisitsakun et al, 2005; Katsuma et al, 2004; Lehiy et al, 2009; Li et al, 2008). Prediction of N-glycosylation sites differs; for example, Bmfgf contains two predicted N-glycosylation sites, Acfgf contains none, and the Spodoptera litura NPV has four (Detvisitsakun et al, 2005; Katsuma et al, 2006a; Katsuma et al, 2004).…”

“…Heparan sulfate proteoglycans are present on the cell surface and their interaction with FGFs tethers them to the surface of the cell membrane. Antibodies detected either vFGF or epitope-tagged vFGF anchored to the surface of the cell and the budded virus envelope (Lehiy et al, 2009; Li et al, 2008). It was hypothesized that when the budded virus acquires its envelope from the vFGF-dotted cell membranes, it incorporates vFGF on its envelope (Lehiy et al, 2009).…”

“…Antibodies detected either vFGF or epitope-tagged vFGF anchored to the surface of the cell and the budded virus envelope (Lehiy et al, 2009; Li et al, 2008). It was hypothesized that when the budded virus acquires its envelope from the vFGF-dotted cell membranes, it incorporates vFGF on its envelope (Lehiy et al, 2009). It is not clear whether vFGF has a function on the budded virus surface or if this association is simply a consequence of its secretion to the cell surface.…”

“…It is not clear whether vFGF has a function on the budded virus surface or if this association is simply a consequence of its secretion to the cell surface. Nevertheless, vFGF on the budded virus has activity; vFGF-carrying budded virions are able to stimulate cell motility better than virions without vFGF (Lehiy et al, 2009; Li et al, 2008). One can envision advantageous scenarios for this, including the ability of vFGF-laden viruses or vFGF cleaved from virions to attract hemocytes or tracheal cells, or a function at late times during budded virus production when the synthesis of vFGF may have declined.…”