HERPES-SIMPLEX VIRUS GD AND VIRIONS ACCUMULATE IN ENDOSOMES BY MANNOSE 6-PHOSPHATE-DEPENDENT AND 6-PHOSPHATE-INDEPENDENT MECHANISMS

Herpes simplex virus (HSV) glycoprotein D (gD) is modified with mannos e 6-phosphate (M6P) and binds to M6P receptors (MPRs), MPRs are involv ed in the well characterized pathway which lysosomal enzymes are direc ted to lysosomes via a network of endosomal membranes, Based on the im paired ability of HSV to form plaques under conditions in which glycop roteins could not interact with MPRs, we proposed that MPRs may functi on during HSV egress or cell-to-cell spread (C. R. Brunetti, R. L. Bur ke, D. Hoflack, T. Ludwig, K. S. Dingwell, and D. C. Johnson, J. Virol . 69:3517-3528, 1995). To further analyze M6P modification and intrace llular trafficking of gD in the absence of other HSV proteins, adenovi rus (Ad) vectors were used to express soluble and membrane-anchored fo rms of gD. Both membrane-bound and soluble gD were modified with M6P r esidues and were localized to endosomes that contained the 275-kDa MPR or the transferrin receptor, Similar results ct ere observed in HSV-i nfected cells, Cell fractionation experiments showed that gD was not p resent in lysosomes. However, a mutant form of gD and another HSV glyc oprotein, gI, that were not modified with M6P were also found in endos omes in HSV-infected cells, Moreover, a substantial fraction of the HS V nucleocapsid protein VP6 was found in endosomes, consistent with acc umulation of virions in an endosomal compartment, Therefore, it appear s 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.