513 Convergence of model systems for peroxisome biogenesis Suresh Subramani Receptors for the two peroxisomal targeting signals PTS1 and PTS2 have recently been identified in yeasts. The disparate subcellular locations of PTS receptor homologs have led to proposals of receptor shuttling between the cytosol and the peroxisomal membrane, and to the suggestion that some of these molecules may even reside normally in the peroxisomal matrix. A peroxisomal membrane protein that interacts with the PTS1 receptor in Saccharomyces cerevisiae may serve as the receptor-docking site on the organelle. The conservation of the PTS1 receptor in several yeasts has led to the cloning and characterization of the gene encoding its human homolog, PTS1 R, which is mutated in a group of patients afflicted with fatal peroxisomal disorders. The identification of peroxisome assembly genes in yeasts is likely to lead to the cloning of additional human genes responsible for other generalized peroxisomal disorders. Protein unfolding is not a prerequisite for import of peroxisomal matrix proteins, suggesting novel mechanisms for the translocation of polypeptides across the peroxisomal membrane. Address Department of Biology, University of California at San Diego, Rm 4314 Bonner Hall, 9500 Gilman Drive, La Jolla, CA 92093-0322, USA; e-mail: ssubramani@ucsd.edu Current Opinion in Cell Biology 1996, 8:513-518 © Current Biology Ltd ISSN 0955-06?4 Abbreviations GFP hsp mPTS PAF Pas Peb Per PMP Pp PTS PTS1 R PXR RCDP Sc YIPay green fluorescent protein heat shock protein peroxisomal membranetargeting signal peroxisome assembly factor peroxisome assembly peroxisome biogenesis peroxisome peroxisomal membrane protein Pichia pastoris peroxisomaltargeting signal PTS1 receptor peroxisometargeting signal receptor rhizomelic chondrodysplasiapunctata Saccharomyces cerevisiae peroxisome assemblyin Yarrowia lipolytica Introduction: multiple model systems for the study of peroxisome biogenesis Understanding the mechanism of peroxisomal protein import is germane to the mechanism of protein trans- location across biological membranes and to human disease. Patients who succumb to any one of several fatal peroxisomal disorders -- Zellweger syndrome, neo- natal adrenoleukodystrophy, infantile Refsum's disease; hyperpipecolic acidemia or rhizomelic chondrodysplasia punctata (RCDP)--are impaired in their ability to import peroxisomal matrix proteins into the lumen of the organelle [1-3,4"]. Cell lines derived from such patients have been propagated and exploited in cell fusion studies, and at least 10 different complementation groups affecting peroxisomal protein import and biogenesis in humans have been defined [5]. This rich genetic resource of mutant cells deficient in peroxisome assembly has been expanded significantly by the development of several yeast model systems in which peroxisomal import and/or biogenesis is compromised [6-14]. The success of these studies stems in large measure from the fact that peroxisomes are apparently dis- pensable under certain conditions, allowing the isolation of non-conditional mutants (known as pas, per, pat, or peb). A close parallel exists among the general classes of import-deficient phenotypes observed in mutant yeast and human cells. The evolutionary conservation of perox- isomal targeting signals (PTSs) and peroxisome biogenesis genes has led to a convergence of model systems that promises to yield new insights into peroxisome function and biogenesis and the role of these processes in human disease. In this review, I focus primarily on new results published in 1995 and 1996. Targeting signals Peroxisomal matrix, in addition to membrane, proteins are necessarily encoded by the nuclear genome, and are imported post-translationally to the organelle. None of the peroxisomal profeins studied to date are glycosylated, nor are they burdened by any modifications that are sometimes necessary for import into organelles [5]. Peroxisomes are no exception to the recurring theme that multiple import pathways are involved in the targeting of proteins to a specific subcellular organelle. Several signals direct protein traffic to the peroxisome. Prominent among these is PTS1, a conserved, carboxy-terminal tripeptide (with the amino acid sequence SKL or a variant thereof) that constitutes the major targeting signal for polypeptides that are en route to the peroxisomal matrix [5]. A second sequence, PTS2, is composed of a conserved nonapeptide with the sequence (R/K)(L/V/I)(Xs)(H/Q)(L/A), and it is used by a smaller subset of matrix proteins [5]. Although naturally located at the amino terminus of proteins, the PTS2 sequence also functions at internal locations in passenger proteins (KN Faber, S Subramani, unpublished data). Other sequences, located internally within peroxisomai matrix proteins such as Candida tropicalis acyl CoA oxidase [15] and Saccharomyces cerevisiae catalase [16] and carnitine acetyl transferase [17]; have been described but their generality is undocumented. Recently, another PTS has been described that is near