The peroxisomeis an important organelle involved in aspects of the metabolism of many molecules, including fatty acids and other lipids (eg, plasmalogens, cholesterol, and bile acids), purines, amino acids, and hydrogen peroxide. The peroxisome is bounded by a single membrane and contains more than 50 enzymes; catalase and urate oxidase are marker enzymes for this organelle. Its proteins are synthesized on cytosolic poly ribosomes and fold prior to import. The pathways of import of a number of its proteins and enzymes have been studied, some being matrix components (Figure 1) and others membrane components. At least two peroxisomal–matrix targeting sequences (PTSs) have been discovered. One, PTS1, is a tripeptide (ie, Ser-Lys-Leu [SKL], but variations of this sequence have been detected) located at the carboxyl terminal of a number of matrix proteins, including catalase. Proteins containing PTS1 sequences form complexes with a cytosolic receptor protein (Pex5) which then dock with a peroxisomal membrane receptor complex containing Pex13, 14, and 17, and the cargo protein is translocated into the lumen. The Pex5 receptor remains in the membrane and is recycled to the cytosol by a process involving other Pexs, including complexes of Pex2, 10, and 12 and Pex1, 6, and 15. A second peroxisome matrix targeting sequence, PTS2, is a nine amino acid sequence at the N-terminus and has been found in at least four matrix proteins (eg, thiolase). Proteins containing PTS2 sequences are known to complex with the receptor protein Pex7 rather than Pex5, but the exact translocation mechanism is less well understood than for PTS1. The PTS1 and PTS2 signals are not cleaved after entry into the matrix.

Fig1. Entry of a protein into the peroxisomal matrix.The protein for import into the matrix (eg, catalase) is synthesized on cytosolic polyribosomes, assumes its folded shape prior to import, and contains a C-terminal peroxisomal-targeting sequence (PTS1) which docks with the cytosolic receptor protein Pex5 and the complex interacts with a receptor on the peroxisomal membrane consisting of Pex13, 14, and 17 (Pex13/14/17). A transient pore is then formed in the membrane and the protein is translocated into the peroxisome matrix. Pex 5 remains in the membrane and is recycled to the cytosol with the aid of complexes of Pex 2, 10, and 12 (Pex2/10/12) and 1, 6, and 15 (Pex 1/6/15). The protein retains PTS1 in the matrix.

Most peroxisomal membrane proteins have been found to contain neither of the two targeting sequences, but seem to contain others. The import system can handleintact oligomers (eg, tetrameric catalase). Import of matrix proteins requires ATP, whereas import of membrane proteins does not.

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