For sucrose gradient analysis, concentrated RSPs were ultracentrifuged in a 20 to 60% discontinuous sucrose gradient at 28,000 rpm (Beckman SW-41Ti rotor) for 2.5 hours in 4uC. All sucrose solutions were prepared with HEPES buffer (20 mM). Fractions of 0.5 ml were collected and levels of E were measured using the CLDB assay. Alternatively, RSPs were treated with 0.5% Triton X-100 for 1 hour before sucrose gradient fractionation, for RSPs denaturation.