PAGE analysis. A: Non denaturing-PAGE of lipid-free apoA-I (in the absence of SDS). Lane 1: 30 μg of WT apoA-I; lane 2: 30 μg S36A apoA-I. The arrow indicates the position of monomeric apoA-I. B: SDS-PAGE of apoA-I cross-linked with DMS. The molecular weight of the marker proteins (M) are shown on the left. Lane 1: 10 μg of apoA-I in the absence of cross-linker. Lane 2: apoA-I (10 μg) and 5 μg DMS, 20 μg DMS (lane 3), and 50 μg DMS (lane 4). WT apoA-I is shown in the left lanes, S36A apoA-I is shown in the right lanes. apoA-I, apolipoprotein A-I; DMS, dimethylsuberimidate; WT, wild-type.