Is the 135S poliovirus particle an intermediate during cell entry?

Poliovirus binding to its receptor (PVR) on the cell surface induces a conf ormational transition which generates an altered particle with a sedimentat ion value of 135S versus the 160S of the native virion. A number of lines o f evidence suggest that the 135S particle is a cell entry intermediate. How ever, the low infection efficiencies of the 135S particle and the absence o f detectable 135S particles during infection at 26 degrees C by the cold-ad apted mutants argue against a role for the 135S particle during the cell en try process. We show here that binding of 135S-antibody complexes to the Fc receptor (CDw32) increases the infectivity of these particles by 2 to 3 or ders of magnitude. Thus, the low efficiency of infection by 135S particles is due in part to the low binding affinity of these particles. In addition, we show that there is an additional stage in the entry process that is ass ociated with RNA release. This stage occurs after formation of the 135S par ticle, is rate limiting during infection at 37 degrees C, but not at 26 deg rees C, and is PVR independent. The data also demonstrate that during infec tion at 26 degrees C, the rate-limiting step is the PVR-mediated conversion of wild-type 160S particles to 135S particles. This suggests that during i nfection at 26 degrees C by the cold-adapted viruses, 135S particles are fo rmed, but they fail to accumulate to detectable levels because the subseque nt post-135S particle events occur at a significantly faster rate than the initial conversion of 160S to 135S particles. These data support a model in which the 135S particle is an intermediate during poliovirus entry.