Abstract
In mitochondria, β-barrel outer membrane proteins mediate protein import, metabolite transport, lipid transport, and biogenesis. The Sorting and Assembly Machinery (SAM) complex consists of three proteins that assemble as a 1:1:1 complex to fold β-barrel proteins and insert them into the mitochondrial outer membrane. We report cryoEM structures of the SAM complex from Myceliophthora thermophila, which show that Sam50 forms a 16-stranded transmembrane β-barrel with a single polypeptide-transport-associated (POTRA) domain extending into the intermembrane space. Sam35 and Sam37 are located on the cytosolic side of the outer membrane, with Sam35 capping Sam50, and Sam37 interacting extensively with Sam35. Sam35 and Sam37 each adopt a GST-like fold, with no functional, structural, or sequence similarity to their bacterial counterparts. Structural analysis shows how the Sam50 β-barrel opens a lateral gate to accommodate its substrates.
Highlights
In mitochondria, β-barrel outer membrane proteins mediate protein import, metabolite transport, lipid transport, and biogenesis
The Sorting and Assembly Machinery (SAM) complex was solubilized from isolated mitochondria using the detergent lauryl maltose neopentyl glycol (LMNG) and purified by affinity chromatography using a Twin-Strep tag on the Nterminus of Sam[37]
Precursor proteins are recognized by the SAM or bacterial assembly machinery (BAM) folding machinery through a sorting signal in the final β-strand, called the β-signal in mitochondria[3] and the C-terminal signature sequence in bacteria[40]
Summary
Β-barrel outer membrane proteins mediate protein import, metabolite transport, lipid transport, and biogenesis. Bacterial β-barrel membrane proteins take a different pathway to reach the outer membrane, but they are inserted by evolutionarily related machinery[9] (Supplementary Fig. 1). A homology model based on BamA was used to study mitochondrial membrane protein insertion by crosslinking precursor proteins to predicted strands β1 and β16 of Sam[50], suggesting that substrates enter the lumen of Sam[50], accumulate at the lateral gate, and are released into the compressed membrane adjacent to the gate[28]. Structures of Tom4030,31 and VDAC32 reveal very similar 19-stranded β-barrel proteins; Mdm[10] is expected to adopt the same fold It appears that Sam[50] substrates (other than Sam[50] itself) may use a single folding and insertion mechanism. The peripheral SAM components, Sam[35] and Sam[37], sit on the opposite side of the membrane and cannot make the Precursor protein
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
