Poliovirus contains a virus particle devoid of a
lipid envelope that does not require an intact pH to enter into susceptible cells. Thus, the blockade of pH gradient generated in endosomes is not sufficient to impede the translocation of poliovirus particles to the cytoplasm, suggesting that translocation takes place at the plasma membrane. Measuring both
viral protein synthesis and eIF4G-1 cleavage mediated by
poliovirus protease 2A has been used to monitor productive entry of poliovirus into cells. Translation of the input poliovirus
RNA produces enough 2A(pro) to cleave eIF4G-1, providing a sensitive assay to estimate poliovirus
RNA delivery to the cytoplasm followed by its translation. Combination of
concanamycin A, a vacuolar
proton-
ATPase inhibitor, and
valinomycin, an
ionophore that promotes K(+) efflux from cells, powerfully prevented poliovirus
infection. Moreover, modifying the ionic conditions of the culture medium (increasing the concentration of K(+) and decreasing the concentration of Na(+)), together with
concanamycin A, also significantly interfered with poliovirus entry. These findings suggest that poliovirus
RNA requires an intact concentration of K(+)
ions inside the cells to be uncoated and to gain access to the cytoplasm. In addition, the actual contribution of
concanamycin A (as well as other inhibitors of endocytosis) to the total inhibition of productive poliovirus entry points to the idea that at least some percentage of polioviral subparticles translocates from the endosomes.