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Cover  Peroxisomes are essential organelles found in virtually all eukaryotic cells. In man, peroxisomes house a variety of enzymes that carry out critical metabolic processes, including the -oxidation of very-long-chain fatty acids, plasmalogen and cholesterol synthesis, and the processing of certain nitrogenous wastes. The organelle contains both hydrogen peroxide-producing oxidases and the hydrogen peroxide-degrading enzyme, catalase.   Peroxisomal protein import is a post-translational signal-mediated event. All but a select few proteins destined for the peroxisome lumen contain a carboxy-terminal targeting signal, called PTS1. In most cases, PTS1 is a tripeptide related to serine-lysine-leucine (SKL). Catalase, a peroxisomal antioxidant enzyme, contains a distinct carboxy-terminal consisting of the four amino acids, lysine-alanine-asparagine-leucine (KANL). Both the SKL and KAML sequences are recognized by the cycling receptor, Pex5p, although to apparently different extents. Concomitant with differential binding to the PTS1 import receptor are differences in the import efficiencies of the two signal variantsan effect especially manifest in aged cells. This month's cover shows two fluorescent reporter proteins: DsRed targeted with the SKL PTS1 and GFP targeted with the catalase (KANL) PTS1, expressed in a senescent human fibroblast. The DsRED-SKL is imported into peroxisomes, while the GFP-KANL remains largely in the cytosol. In these cells, discordant peroxisomal import efficiencies may result in the net production of hydrogen peroxide. (Legakis et al. (2002), Mol. Biol. Cell 13, 4243-4255).Paul A. Walton and Stanley R. Terlecky