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PEROXISOME BIOGENESIS
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The essential features of peroxisome biogenesis have been elucidated in recent years. These features are: 1) peroxisomes are not autonomously multiplying organelles but are derived from the endoplasmic reticulum; 2) all peroxisomal proteins, including peroxisomal matrix and membrane proteins, are encoded by the nuclear genome and synthesized on free polyribosomes; 3) the newly synthesized proteins are post-translationally imported from the cytosol into preexisting peroxisomes; and 4) import of new peroxisomal proteins into peroxisomes leads to an expansion of the size of peroxisomes, which makes them grow until a critical size is reached. Subsequently, the peroxisomes divide into two daughter peroxisomes that can then undergo the same cycle of events. Figure 24-1 presents a simplified scheme of peroxisome biogenesis. Correct targeting of proteins to peroxisomes is achieved via so-called peroxisome targeting signals (PTSs). Peroxisomal matrix proteins are targeted to peroxisomes via one of two different targeting signals which are short, conserved stretches of amino acids at the C-terminal (PTS1) or N-terminal (PTS2) end of peroxisomal matrix proteins. Several relevant proteins that possess PTS1 or PTS2 proteins are listed in Table 24-1. Proteins with PTS sequences are recognized in the cytosol by receptor proteins. The loaded receptors are then recognized specifically by the peroxisomal protein import machinery, after which the matrix proteins are translocated across the peroxisomal membrane, whereas the receptors are released back into the cytosol for another round of import. In principle, peroxisomal membrane proteins follow a similar pathway, although the targeting signal is different as well as the receptor recognizing this signal (Figure 24-1).
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