We have shown that foot-and-mouth disease virus (FMDV)
infection mediated by the
integrin alphavbeta6 takes place through
clathrin-dependent endocytosis but not caveolae or other endocytic pathways that depend on
lipid rafts. Inhibition of
clathrin-dependent endocytosis by
sucrose treatment or expression of a dominant-negative version of AP180 inhibited virus entry and
infection. Similarly, inhibition of endosomal acidification inhibited an early step in
infection. Blocking endosomal acidification did not interfere with surface expression of alphavbeta6, virus binding to the cells, uptake of the virus into endosomes, or cytoplasmic virus replication, suggesting that the low pH within endosomes is a prerequisite for delivery of
viral RNA into the cytosol. Using immunofluorescence confocal microscopy, FMDV colocalized with alphavbeta6 at the cell surface but not with the B subunit of
cholera toxin, a marker for
lipid rafts. At 37 degrees C, virus was rapidly taken up into the cells and colocalized with markers for early and recycling endosomes but not with a marker for lysosomes, suggesting that
infection occurs from within the early or recycling endosomal compartments. This conclusion was supported by the observation that FMDV
infection is not inhibited by
nocodazole, a
reagent that inhibits vesicular trafficking between early and late endosomes (and hence trafficking to lysosomes). The
integrin alphavbeta6 was also seen to accumulate in early and recycling endosomes on virus entry, suggesting that the
integrin serves not only as an attachment receptor but also to deliver the virus to the acidic endosomes. These findings are all consistent with FMDV
infection proceeding via
clathrin-dependent endocytosis.