Abstract
Mitochondria are organelles derived from an intracellular α-proteobacterium. The biogenesis of mitochondria relies on the assembly of β-barrel proteins into the mitochondrial outer membrane, a process inherited from the bacterial ancestor. Caulobacter crescentus is an α-proteobacterium, and the BAM (β-barrel assembly machinery) complex was purified and characterized from this model organism. Like the mitochondrial sorting and assembly machinery complex, we find the BAM complex to be modular in nature. A ∼150 kDa core BAM complex containing BamA, BamB, BamD, and BamE associates with additional modules in the outer membrane. One of these modules, Pal, is a lipoprotein that provides a means for anchorage to the peptidoglycan layer of the cell wall. We suggest the modular design of the BAM complex facilitates access to substrates from the protein translocase in the inner membrane.
Highlights
Mitochondria are essential organelles, derived in the first eukaryotes from intracellular bacterial symbionts
Whereas the mitochondrial Sam50 has a truncated N-terminal domain (Figure 1A), the BamA protein from C. crescentus and other a-proteobacteria conform to the basic domain structure seen in all other BamA proteins: a signal sequence for secretion via the SecYEG machinery, five predicted POTRA domains and a C-terminal b-barrel that would be embedded in the outer membrane (Figure 1A)
In C. crescentus, TonB-dependent receptors appear to be the dominant means of substrate transport across the outer membrane and the C. crescentus genome codes for an extraordinary sixty-seven TonB-dependent receptors, leading to the suggestion that a repertoire of diverse active transport pathways can be established in the outer membrane of this bacterium [37,38]
Summary
Mitochondria are essential organelles, derived in the first eukaryotes from intracellular bacterial symbionts. Protein subunits that are important, but not essential, for the core function of b-barrel assembly can be titrated from the complex by increasing the concentration of detergent used in purification. In this way the essential protein ‘‘modules’’ such as Mdm and Mim, can be selectively removed from the holo-SAM complex [5,6,7,8,9,10]. This modular machine in the outer membrane receives its substrates via small TIM chaperones that link the protein translocation device (the TOM complex) with the SAM complex [11,12]
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