The Mpv17-like protein (M-LP) 1 gene was identified on the basis of an expressed sequence tag obtained by differential display screening of age dependently expressed genes in mouse kidneys (1, 2). The M-LP gene is expressed mainly in the kidney and spleen and the amount expressed increases steadily during development, reaches its highest level in adulthood, and decreases gradually with aging. It encodes 194 amino acids of a polypeptide with a sequence and membrane topology markedly similar to those of two peroxisomal membrane proteins, Mpv17 protein (3) (30.4% identity, 66.7% similarity) and PMP22 (4) (25.0% identity, 72.1% similarity). These results suggest that M-LP might be embedded in the peroxisomal membrane in a similar manner to these two proteins.Peroxisomes are ubiquitous eukaryotic subcellular organelles that play essential roles in a variety of metabolic pathways, including H 2 O 2 metabolism and the oxidative degradation of fatty acids (5). Recently, a number of studies have been carried out to investigate the mechanisms of peroxisomal biogenesis and protein import (6 -8). Targeting of peroxisomal matrix proteins is mediated by cytosolic receptors Pex5p and Pex7p that recognize the peroxisomal targeting signals PTS1, which consists of the sequence SKL (and conservative variants) at the carboxyl terminus (9, 10), and PTS2, which consists of the consensus sequence (R/K)(L/V/I)X 5 (H/Q)(L/A) near the amino terminus (11, 12), respectively. Whereas the targeting of peroxisomal matrix protein import has been well characterized, neither conclusive consensus sequence nor the receptor(s) involved in peroxisomal membrane protein targeting have been determined. Thus, it is important to characterize the membrane peroxisome targeting signals (mPTSs) in as many peroxisomal membrane proteins as possible to understand the biogenesis of peroxisomal membranes.The Mpv17-negative mouse strain was generated by inserting a defective retrovirus into the germ line of mice and is characterized by progressive glomerulosclerosis and neurosensory deafness at a young age (3, 13). The phenotype results from loss of function of the Mpv17 gene encoding a 20-kDa peroxisomal membrane protein. The molecular function of the Mpv17 protein has yet to be elucidated, but it was recently hypothesized that it plays an important role in peroxisomal reactive oxygen species (ROS) metabolism and that glomerular damage is because of overproduction of ROS. In Mpv17 geneinactivated mice, a significant increase in ROS and lipid peroxidation adduct production was observed and oxygen radical scavengers prevented glomerular damage (14). Moreover, a recent study showed that the ␥-glutamyl transpeptidase enzyme activity and mRNA expression level were higher, whereas plasma glutathione peroxidase (Gpx3) and superoxide dismutase (SOD) activities were lower, in Mpv17 null cells than normal cells (15). These results strongly suggest that the Mpv17 protein is involved in enzymatic antioxidant defense systems. The aims of this study were first, to confirm the ...