The outer membranes of mitochondria have b-barrel proteins and these are assembled by the SAM complex. An (+/-)-Sulfinpyrazone essential feature of the SAM complex is its modular nature. 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 Mdm10 and Mim1, can be selectively removed from the holo-SAM complex. This modular machine in the outer membrane receives its substrates via small TIM chaperones that link the protein translocation device with the SAM complex. A central and essential component of the mitochondrial SAM complex, Sam50, is a member of the Omp85 family of proteins. All bacteria with outer membranes have an Omp85 protein, with studies in both Neisseria meningitidis and Escherichia coli showing that the Omp85 protein mediates the assembly of b-barrel proteins into the outer membrane. Thus, mitochondria have retained from their a-proteobacterial ancestry the machinery to assemble proteins into the organelle��s outer membrane. However, there are significant differences in the subunit composition of the mitochondrial SAM complex and the BAM complex in the model c-proteobacterium E. coli. Recent studies in E. coli have shown that the Omp85 protein, now referred to as BamA, is the central component of a complex that also contains four lipoprotein partners: BamB, BamC, BamD and BamE. BamB interacts with BamA and the interaction site has been mapped by mutagenesis ; BamB is predicted to have a b-propeller structure and the interaction might be mediated through Kuwanone H unpaired b-strands in BamA and BamB. In Serratia marcescens, BamC has been implicated in the regulation of cell motility, but little is known of its function. The gene encoding BamD is essential for cell viability in E. coli and BamD and BamC interact directly.