Biogenesis of αlpha-helical outer membrane proteins
Abstract
Mitochondrial proteins are synthezised on cytosolic ribosomes and imported into the organelle via different proteinaceous machineries. The translocase of the outer membrane (TOM complex) forms the entry gate for the majority of precursor proteins. Subsequently, the precursors are sorted into the different subcompartments like inner membrane, intermembrane space and into the matrix. Even outer membrane proteins with beta-barrel structure are first transported across the TOM machinery and then inserted into the outer membrane via the sorting and assembly machinery (SAM complex). In contrast, only little is known how outer membrane proteins with alpha-helical membrane anchor reach their final destination. Here, we studied the biogenesis of multispanning outer membrane proteins like Ugo1. We used a novel approach to search for proteins involved in their biogenesis. The import of His-tagged chemical amounts saturates the import system that result in an accumulation of import intermediates that can be purified utilizing the His-tag. Following this strategy we could show that the precursor binds to the TOM complex and utilizes Tom70 as entry receptor. Strikingly, oligomeric structures of Mim1 interact with the precursor and Mim1 is a critical component for the assembly of Ugo1. In addition, we found that Mim1 is also a biogenesis factor of some Tom subunits with a single alpha-helical membrane-span. Taken all together, we show for the first time components of the biogenesis pathway of outer membrane proteins with alpha-helical membrane spans.
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