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Proteins that should function inside the mitochondria or chloroplasts are synthesized as precursor proteins on cytosolic ribosomes and carry a recognition sequence on the N‐terminal (Table 5.1). After uptake by the organelle, this signal sequence is removed by a signal peptidase. The import progresses via a multienzyme complex: the translocase of outer membrane(TOM) complex binds a precursor protein and transports it over the outer mitochondrial membrane. Further transport over the inner mitochondrial membrane is taken over by TIM22 and TIM23 complexes (Figure 5.4). When membrane proteins are imported, they contain an additional signal sequence, which is then recognized by the OXA complex. The OXA complex ensures that membrane proteins, whether synthesized by the mitochondria or imported out of the cytosol, are incorporated correctly in the inner mitochondrial membrane. A SAM complex helps to place proteins in the outer mitochondrial membrane.

Figure 5.4 Schematic overview of the uptake of a precursor protein by the mitochondria and the assembly of membrane proteins in the inner mitochondrial membrane. Eukarya, translocase of outer membranes; TIMs, translocase of inner membranes. (a) Setup of transport systems. (b) Cooperation between Eukarya and TIM complexes. (c) Function of the OXA complex.

Transporting precursor proteins into chloroplasts follows a similar scheme. A second signal is necessary for transport into the thylakoid.

Peroxisomes harbor enzymes to break down hydrogen peroxide (catalase) and the enzymes of β‐oxidation of fatty acids. Proteins targeted for peroxisomes carry a short signal peptide of three amino acids (Ser‐Lys‐Leu) on their C‐terminus. Peroxisomes carry a complex of protein translocators, peroxins (e.g. Pex1, Pex5, Pex6, Pex7), which are activated by adenosine triphosphate (ATP).