FIG. 3.

Localization and morphology of MuERV-L-associated viruslike particles. (A) Confocal images of human cells transfected with the MuERV-L vector or a control plasmid (ctrl). Forty-eight hours posttransfection, cells were fixed, permeabilized, and stained with the anti-Gag MuERV-L antiserum and an Alexa fluor 488-conjugated antirabbit immunoglobulin G secondary antibody (in green). Nuclei were stained with TO-PRO-3 iodide (in blue). (B) Electron microscopy of mouse epsilon particles. (1) Representative low-magnification image of 293T cells transfected with MuERV-L disclosing particles accumulated in the cisternae of the ER (brackets). No particles can be observed at the plasma membrane (Pm). ERm, ER membrane; Nu, nucleus; M; mitochondria. (2) High-magnification images of 293T cells transfected with MuERV-L disclosing particles within or budding into the ER. (3) High-magnification image of a naturally occurring epsilon particle budding into the ER of a mouse embryo, and schematic structure of an epsilon particle (inset, adapted with permission from reference 13).