Two distinct binding affinities of poliovirus for its cellular receptor

To study the kinetics and equilibrium of poliovirus binding to the poliovir us receptor, we used surface plasmon resonance to examine the interaction o f a soluble form of the receptor with Poliovirus. Soluble receptor purified from mammalian cells is able to bind poliovirus, neutralize viral infectiv ity, and induce structural changes in the virus particle, Binding studies r evealed that there are two binding sites for the receptor on the poliovirus type 1 capsid, with affinity constants at 20 degrees C of K-D1 = 0.67 mu M and K-D2, = 0.11 mu M. The relative abundance of the two binding sites var ies with temperature. At 20 circle C, the K-D2 site constitutes approximate ly 46% of the total binding sites on the sensor chip, and its relative abun dance decreased with decreasing temperature such that at 5 degrees C, the r elative abundance of the K-D2 site is only 12% of the total binding sites. Absolute levels of the K-D1 site remained relatively constant at all temper atures tested. The two binding sites may correspond to docking sites for do main 1 of the receptor on the viral capsid, as predicted by a model of the poliovirus-receptor complex. Alternatively, the binding sites may be a cons equence of structural breathing, or could result from receptor-induced conf ormational changes in the virus.