Channel activity of deamidated isoforms of prion protein fragment 106-126 in planar lipid bilayers

Using the lipid bilayer technique, we have found that age-related derivatives, PrP[106-126] (L-Asp108) and PrP[106-126] (L-iso-Asp108), of the prion protein fragment 106-126 (PrP[106-126] (Asn108)) form heterogeneous ion channels. The deamidated isoforms, PrP[106-126] (L-Asp108) and PrP[106-126] (L-iso-Asp108), showed no enhanced propensity to form heterogeneous channels compared with PrP[106-126] (Asn108). One of the PrP[106-126] (L-Asp108)- and PrP[106-126] (L-iso-Asp108)-formed channels had three kinetic modes. The current-voltage (I-V) relationship of this channel, which had a reversal potential, E_rev, between -40 and -10 mV close to the equilibrium potential for K⁺ (E_K -35 mV), exhibited a sigmoidal shape. The value of the maximal slope conductance (g_max) was 62.5 pS at positive potentials between 0 and 140 mV. The probability (P_o) and the frequency (F_o) of the channel being open had inverted and bell-shaped curves, respectively, with a peak at membrane potential (V_m) between -80 and +80 mV. The mean open and closed times (T_o and T_c) had inverted bell-shaped curves. The biophysical properties of PrP[106-126] (L-Asp108)- and PrP[106-126] (L-iso-Asp108)-formed channels and their response to Cu²⁺ were similar to those of channels formed with PrP[106-126] (Asn108). Cu²⁺ shifted the kinetics of the channel from being in the open state to a "burst state" in which rapid channel activities were separated by long durations of inactivity. The action of Cu²⁺ on the open channel activity was both time-dependent and voltage-dependent. The fact that Cu²⁺ induced changes in the kinetics of this channel with no changes in the conductance of the channel indicated that Cu²⁺ binds at the mouth of the channel. Consistently with the hydrophilic and structural properties of PrP[106-126], the Cu²⁺-induced changes in the kinetic parameters of this channel suggest that the Cu²⁺ binding site could be located at M₁₀₉ and H₁₁₁ of this prion fragment.