Membrane protein insertion through a mitochondrial β-barrel gate

Abstract

The biogenesis of mitochondria, chloroplasts, and Gram-negative bacteria requires the insertion of β-barrel proteins into the outer membranes. Homologous Omp85 proteins are essential for membrane insertion of β-barrel precursors. It is unknown if precursors are threaded through the Omp85-channel interior and exit laterally or if they are translocated into the membrane at the Omp85-lipid interface. We have mapped the interaction of a precursor in transit with the mitochondrial Omp85-channel Sam50 in the native membrane environment. The precursor is translocated into the channel interior, interacts with an internal loop, and inserts into the lateral gate by β-signal exchange. Transport through the Omp85-channel interior followed by release through the lateral gate into the lipid phase may represent a basic mechanism for membrane insertion of β-barrel proteins.

Mitochondrial Outer Membrane Channels: Emerging Diversity in Transport Processes

“…The β‐signal of the incoming precursor binds to the first β‐strand of Sam50 and thereby replaces the endogenous β‐signal of Sam50. The precursor stays at the lateral gate until the β‐barrel is folded . Sam50 may also stimulate the release of the precursor by distortion of the lipid bilayer as it was reported for the bacterial BamA .…”

“…Strikingly, addition of a β‐signal almost doubled the channel conductance to about 640 pS, indicating that the Sam50 channel pore is highly flexible. A recent study shows that the Sam50 channel is directly involved in protein insertion . Like the bacterial homolog BamA the β‐barrel of Sam50 forms a lateral gate between the first and the last β‐strand .…”

“…A recent study shows that the Sam50 channel is directly involved in protein insertion . Like the bacterial homolog BamA the β‐barrel of Sam50 forms a lateral gate between the first and the last β‐strand . Disulfide bond scanning reveals that the β‐barrel precursors are transferred via the channel lumen into the lateral gate of Sam50.…”

Structural considerations of folded protein import through the chloroplast TOC/TIC translocons

“…There is reason to believe this is possible since the terminal β‐strands of Toc75 are structurally similar to the BamA/Sam50 homologs, which have weakly interacting terminal β‐strands . Recently, the mechanism of β‐barrel insertion via Sam50 was described wherein the first and last β‐strands of Sam50 pair with incoming substrate β‐strands . Once the substrate β‐barrel protein is fully inserted in the membrane, its terminal β‐strands would be paired with the terminal β‐strands of Sam50.…”

Cryo-EM structure of the mitochondrial protein-import channel TOM complex from Saccharomyces cerevisiae

“…The other Tom 39 proteins are associated with Tom40 by their single α-helical transmembrane segments (TMs). Although 40 functions of the α-helical Tom subunits are relatively poorly defined, they have been suggested to act either 41 as receptors for precursor proteins [16][17][18][19][20] or as binding sites for other factors 20,21 , and/or as escorts that 42 promote assembly and stability of the TOM complex 6,10,22,23 . 43 Current evidence indicates that translocation is a sequential process in which a precursor protein is first 44 recruited by the cytosolic receptor domains of Tom70, Tom20, and Tom22, then threaded into the pore of 45 Tom40, and finally handed over to the translocase of the inner membrane (TIM) complex or IMS-resident 46 chaperones (for review, see ref.…”