The PAL1 gene was isolated using PCR and degenerate oligonucleotide primers corresponding to highly conserved amino acid sequence motifs diagnostic of the ATP-binding cassette domain of the superfamily of membrane-bound transport proteins typified by mammalian multidrug resistance transporter 1 and Saccharomyces cerevisiae Ste6. The deduced PAL1 gene product is similar in length to, has the same predicted topology as, and shares the highest degree of amino acid sequence identity with two human proteins, adrenoleukodystrophy protein and peroxisomal membrane protein (70 kD), which are both presumptive ATP-binding cassette transporters thought to be constituents of the peroxisomal membrane. As judged by hybridization of a PAL1 probe to isolated RNA and by expression of a PAL1-lacZ fusion, a PAL1 transcript was only detectable when cells were grown on oleic acid, a carbon source which requires the biogenesis of functional peroxisomes for its metabolism. A pal1delta mutant grew normally on either glucose- or glycerol-containing media; however, unlike PAL1+ cells (or the pal1delta mutant carrying the PAL1 gene on a plasmid), pal1delta cells were unable to grow on either a solid medium or a liquid medium containing oleic acid as the sole carbon source. Antibodies raised against a chimeric protein in which the COOH-terminal domain of Pal1 was fused to glutathione S-transferase specifically recognized a protein in extracts from wild-type cells only when grown on oleic acid; this species represents the PAL1 gene product because it was missing in pal1delta cells and more abundant in pal1delta cells expressing PAL1 from a multicopy plasmid. The Pal1 polypeptide was highly enriched in the organellar pellet fraction prepared from wild-type cells by differential centrifugation and comigrated upon velocity sedimentation in a Nycodenz gradient with a known component of the peroxisomal matrix, e-oxoacyl-CoA thiolase. As judged by both subcellular fractionation and indirect immunofluorescence, localization of 3-oxoacyl-CoA thiolase to peroxisomes was unchanged whether Pal1 was present, absent, or overexpressed. These findings demonstrate that Pal1 is a peroxisome-specific protein, that it is required for peroxisome function, but that it is not necessary for the biogenesis of peroxisomes or for the import of 3-oxoacyl-CoA thiolase (and at least two other peroxisomal matrix proteins).

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