Abstract
The Saccharomyces cerevisiae pex17-1 mutant was isolated from a screen to identify mutants defective in peroxisome biogenesis. pex17-1 and pex17 null mutants fail to import matrix proteins into peroxisomes via both PTS1- and PTS2-dependent pathways. The PEX17 gene (formerly PAS9; Albertini, M., P. Rehling, R. Erdmann, W. Girzalsky, J.A.K.W. Kiel, M. Veenhuis, and W.-H Kunau. 1997. Cell. 89:83-92) encodes a polypeptide of 199 amino acids with one predicted membrane spanning region and two putative coiled-coil structures. However, localization studies demonstrate that Pex17p is a peripheral membrane protein located at the surface of peroxisomes. Particulate structures containing the peroxisomal integral membrane proteins Pex3p and Pex11p are evident in pex17 mutant cells, indicating the existence of peroxisomal remnants ("ghosts"). This finding suggests that pex17 null mutant cells are not impaired in peroxisomal membrane biogenesis. Two-hybrid studies showed that Pex17p directly binds to Pex14p, the recently proposed point of convergence for the two peroxisomal targeting signal (PTS)-dependent import pathways, and indirectly to Pex5p, the PTS1 receptor. The latter interaction requires Pex14p, indicating the potential of these three peroxins to form a trimeric complex. This conclusion is supported by immunoprecipitation experiments showing that Pex14p and Pex17p coprecipitate with both PTS receptors in the absence of Pex13p. From these and other studies we conclude that Pex17p, in addition to Pex13p and Pex14p, is the third identified component of the peroxisomal translocation machinery.
Highlights
Protein transport across the peroxisomal membrane from the cytoplasm into the peroxisomal matrix is thought to occur in a posttranslational manner (Lazarow and Fujiki, 1985)
Two distinct peroxisomal targeting signals (PTSs)1 that provide the molecular basis for the specificity of this process have been identified in peroxisomal matrix proteins
We have previously demonstrated that mutants of S. cerevisiae defective in various aspects of peroxisome function and peroxisome biogenesis can be isolated from a population of cells that were unable to grow on oleic acid as the sole carbon source (Erdmann et al, 1989). pex17-1 was selected from such onu strains by two additional criteria: the mislocalization of peroxisomal marker enzymes to the cytosol and the lack of morphologically detectable peroxisomes as determined by electron microscopy
Summary
Protein transport across the peroxisomal membrane from the cytoplasm into the peroxisomal matrix is thought to occur in a posttranslational manner (Lazarow and Fujiki, 1985). One of them, designated PTS1, is the carboxy-terminal tripeptide SKL, which was first discovered in firefly luciferase (Gould et al, 1989; for review see Subramani, 1993). This tripeptide or variants of it are found in the majority of the known intraperoxisomal proteins. Proteins is targeted to the peroxisomal lumen through an alternative signal termed PTS2 This sorting motif is located close to the amino terminus and has the more complex consensus sequence RLX5H/QL (for review see DeHoop and Ab, 1992; Rehling et al, 1996a). These observations established the existence of at least two distinct import pathways for peroxisomal matrix proteins. Dodt and Gould (1996) have provided data consistent with at least the first step of this cycle
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