The pas8 Mutant of Pichia pastoris Exhibits the Peroxisomal Protein Import Deficiencies of Zellweger Syndrome Cells-The PAS8 Protein Binds to the COOH-Terminal Tripeptide Peroxisomal Targeting Signal, and Is a Member of the TPR Protein Family

We previously described the isolation of mutants of the yeast P~chia pastoris that are deficient in peroxisome assembly (pas mutants). We describe the characterization of one of these mutants, pas8, and the cloning of the PAS8 gene. The pas8 mutant is deficient for growth, but not for division or segregation of peroxisomes, or for induction of peroxisomal proteins. Two distinct peroxisomal targeting signals, PTS1 and PTS2, have been identified that are sufficient to direct proteins to the peroxisomal matrix. We show that the pas8 mutant is deficient in the import of proteins with the PTS1, but not the PTS2, targeting signal. This is the same import deficiency as that found in cells from patients with the lethal human peroxisomal disorder Zellweger syndrome. Cloning and sequencing of the PAS8 gene reveals that it is a novel member of the tetratricopeptide repeat gene family. Antibodies raised against bacterially expressed PAS8 are used to show that PASS is a peroxisomal, membrane-associated protein. Also, we have found that in vitro translated PAS8 protein is capable of binding the PTSl targeting signal specifically, raising the possibility that PAS8 is a PTS1 receptor. T hE compartmentalization of cellular processes into different subcellular compartments is one of the hallmarks of the eukaryotic cell. One such compartment, the peroxisome, is ubiquitously present in virtually all eukaryotic cells. Specialized versions of peroxisome-like compartments (e.g., glycosomes of trypanosomes and glyoxysomes of plants) exist in certain organisms. The peroxisomes, glycosomes, and glyoxysomes have been termed collectively as microbodies. Peroxisomes are characterized by the presence of at least one peroxide-generating oxidase and the enzyme catalase which degrades hydrogen peroxide. Peroxisomes are bounded by a single membrane and vary in size from 0.1 to 1.0 #m in diameter. Like mitochondria, peroxisomes are thought to arise by budding and division of preexisting organelles. Unlike mitochondria, peroxisomes do not contain DNA and must import all constituent proteins. Both peroxisomal matrix and membrane proteins are synthesized in the cytosol on free polysomes and imported posttranslationally into the organdie (for review see Lazarow and Fujiki, 1985). Although the targeting signals for peroxisomal membrane proteins are unknown, recent work has demonstrated the existence of at least two distinct signals that will direct proteins to the peroxisomal matrix (for review see Subramani, 1992). Please address all correspondence to Dr. S. Subramani, Department of Biology, 0322 Bonnet Hall, Room 3226, University of California San Diego, La Joila, CA 92093. The most common peroxisomal targeting signal (PTS1) 1 is typified by the carboxy-terminal signal SKL found in the firefly luciferase protein. Certain conservative substitutions in this signal are allowed in mammalian cells (Gould et al., 1989; Swinkels et al., 1992) and in yeast (Aitchison et al., 1991). The SKL signal can target proteins to microbodies in virtually all eukaryotes from trypanosomes, fungi, and plants to mammals (Gould et al., 1990). Antibodies made against the SKL peptide also recognize microbodies in all eukaryotic species that have been examined (Keller et al., 1991). These results demonstrate that the mechanisms of peroxisomal protein import are likely to be conserved among all eukaryotes. A second peroxisomal targeting signal (PTS2) has been found near the amino terminus of the rat peroxisomal 3-ketoacyl CoA thiolase protein. This signal has been delimited to an ll-amino acid sequence that bears no similarity to the PTS1 signal. Like PTS1, PTS2 can direct heterologous proteins to the peroxisome. Little is known about the evolutionary conservation of PTS2, although it appears to be conserved in several fungal thiolases (Swinkels et al., 1991; 1. Abbreviations used in this paper: DBS, dilution of blocking solution; DHAS, dihydroxyacetone synthase; HSA, human serum albumin; MW, molecular weight; Mut, methanol utilization; PAS, peroxisome assembly; PTS, peroxisome targeting signal; SD, synthetic dextrose; SM, synthetic methanol; SOL, synthetic oleate; SOLT, synthetic oleate and Tween; TPR, tetratricopeptide repeat; YP, yeast extract peptone; YPD, yeast extract peptone dextrose; YPM, yeast extract peptone methanol; YPOLT, yeast extract peptone oleate Tween. © The Rockefeller University Press, 0021-9525/93/05/761/14 $2.00 The Journal of Cell Biology, Volume 121, Number 4, May 1993 761-774 761 on F ebuary 1, 2013 jcb.rress.org D ow nladed fom Published May 15, 1993