Response: The Mitochondrial β-Signal and Protein Sorting

The outer membranes of mitochondria, chloroplasts, and Gram-negative bacteria contain abundant β-barrel proteins that are essential for the transport of proteins and metabolites. Identification of the mitochondrial sorting and assembly machinery (SAM complex) revealed a new protein import pathway and sparked interest in mitochondrial β-barrel biogenesis. A central SAM component, Sam50, is conserved from bacteria to humans and a related protein is also found in chloroplasts, implying a conserved mechanism of β-barrel sorting in eukaryotes and prokaryotes (Pfanner et al., 2004; Paschen et al., 2005; Dolezal et al., 2006; Bos et al., 2007).