Dissecting Membrane Insertion of Mitochondrial β-Barrel Proteins

Communication of mitochondria with the rest of the cell requires β-barrel proteins of the outer membrane. All β-barrel proteins are synthesized as precursors in the cytosol and imported into mitochondria by the general translocase TOM and the sorting machinery SAM. The SAM complex contains two proteins essential for cell viability, the channel-forming Sam50 and Sam35. We have identified the sorting signal of mitochondrial β-barrel proteins that is universal in all eukaryotic kingdoms. The β-signal initiates precursor insertion into a hydrophilic, proteinaceous membrane environment by forming a ternary complex with Sam35 and Sam50. Sam35 recognizes the β-signal, inducing a major conductance increase of the Sam50 channel. Subsequent precursor release from SAM is coupled to integration into the lipid phase. We propose that a two-stage mechanism of signal-driven insertion into a membrane protein complex and subsequent integration into the lipid phase may represent a general mechanism for biogenesis of β-barrel proteins.