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Herpes simplex virus gD and virions accumulate in endosomes by mannose 6-phosphate-dependent and -independent
C R Brunetti1, K S Dingwell, C Wale
1McMaster Cancer Research Group, McMaster University, Hamilton, Ontario, Canada.
Abstract:
Herpes simplex virus (HSV) glycoprotein D (gD) is modified with mannose 6-phosphate (M6P) and binds to M6P receptors (MPRs). MPRs are involved in the well-characterized pathway by which lysosomal enzymes are directed to lysosomes via a network of endosomal membranes. Based on the impaired ability of HSV to form plaques under conditions in which glycoproteins could not interact with MPRs, we proposed that MPRs may function during HSV egress or cell-to-cell spread (C. R. Brunetti, R. L. Burke, B. Hoflack, T. Ludwig, K. S. Dingwell, and D. C. Johnson, J. Virol. 69:3517-3528, 1995). To further analyze M6P modification and intracellular trafficking of gD in the absence of other HSV proteins, adenovirus (Ad) vectors were used to express soluble and membrane-anchored forms of gD. Both membrane-bound and soluble gD were modified with M6P residues and were localized to endosomes that contained the 275-kDa MPR or the transferrin receptor. Similar results were observed in HSV-infected cells. Cell fractionation experiments showed that gD was not present in lysosomes. However, a mutant form of gD and another HSV glycoprotein, gI, that were not modified with M6P were also found in endosomes in HSV-infected cells. Moreover, a substantial fraction of the HSV nucleocapsid protein VP6 was found in endosomes, consistent with accumulation of virions in an endosomal compartment. Therefore, it appears that HSV glycoproteins and virions are directed to endosomes, by M6P-dependent as well as by M6P-independent mechanisms, either as part of the virus egress pathway or by endocytosis from the cell surface.
Insights
Herpes simplex virus glycoprotein D (gD) uses mannose 6-phosphate (M6P) and M6P receptors (MPRs) for trafficking to endosomes. Both M6P-dependent and independent pathways direct HSV components to endosomes during virus spread.
Area of Science:
- Virology
- Cell Biology
- Molecular Biology
Background:
- Herpes simplex virus (HSV) glycoprotein D (gD) is modified with mannose 6-phosphate (M6P).
- M6P modification targets proteins to mannose 6-phosphate receptors (MPRs), crucial for lysosomal enzyme trafficking.
- Previous studies suggested MPR involvement in HSV egress or cell-to-cell spread.
Purpose of the Study:
- To investigate M6P modification and intracellular trafficking of HSV gD independently of other viral proteins.
- To determine the role of M6P-dependent and independent pathways in HSV component localization.
Main Methods:
- Adenovirus (Ad) vectors were used to express soluble and membrane-anchored forms of gD.
- Cells were analyzed for M6P modification, receptor binding, and subcellular localization using cell fractionation.
- HSV-infected cells were examined for the localization of gD, gI, and VP6.
Main Results:
- Both soluble and membrane-bound gD were modified with M6P and localized to endosomes containing MPRs or transferrin receptor.
- gD was not detected in lysosomes.
- M6P-independent mechanisms also directed gD, gI, and nucleocapsid protein VP6 to endosomes, suggesting virion accumulation.
Conclusions:
- HSV glycoproteins and virions are directed to endosomes via both M6P-dependent and M6P-independent pathways.
- These pathways may be involved in virus egress or endocytosis from the cell surface.
- Endosomal localization of viral components is a significant aspect of the HSV life cycle.