(A) Model of the Sam50 β-barrel. β-Strand 15, purple; open and filled circles, residues facing the interior of the barrel (black) or the lipid phase (gray), respectively. (B and C) Radiolabeled Por1 precursor variants were imported into mitochondria of yeast strains expressing the indicated Sam50 variants. Samples were crosslinked with BMH, AMAS or SIA and analyzed by non-reducing SDS-PAGE and autoradiography. Black and white arrowheads, cysteine-specific precursor adducts to Sam50-β16 or Sam50-β15, respectively; circles, unspecific/cysteine-independent adducts (in the absence of free SH-groups, AMAS and SIA can react with other amino acids).