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CELL BIOLOGY AND SIGNAL TRANSDUCTION

Characterization of the Targeting Signal of the Arabidopsis 22-kD Integral Peroxisomal Membrane Protein^1,[w]

Department of Botany, University of Guelph, Guelph, Ontario, Canada N1G 2W1 (M.A.M., R.T.M.); and Centre for Plant Sciences, University of Leeds, Leeds, United Kingdom LS2 9JT (B.A.P., A.B.).

Using a combination of in vivo and in vitro assays, we characterized the sorting pathway and molecular targeting signal for the Arabidopsis 22-kD peroxisome membrane protein (PMP22), an integral component of the membrane of all peroxisomes in the mature plant. We show that nascent PMP22 is sorted directly from the cytosol to peroxisomes and that it is inserted into the peroxisomal boundary membrane with its N- and C-termini facing the cytosol. This direct sorting of PMP22 to peroxisomes contrasts with the indirect sorting reported previously for cottonseed (Gossypium hirsutum) ascorbate peroxidase, an integral PMP that sorts to peroxisomes via a subdomain of the endoplasmic reticulum. Thus, at least two different sorting pathways for PMPs exist in plant cells. At least four distinct regions within the N-terminal one-half of PMP22, including a positively charged domain present in most peroxisomal integral membrane-destined proteins, functions in a cooperative manner in efficient peroxisomal targeting and insertion. In addition, targeting with high fidelity to peroxisomes requires all four membrane-spanning domains in PMP22. Together, these results illustrate that the PMP22 membrane peroxisomal targeting signal is complex and that different elements within the signal may be responsible for mediating unique aspects of PMP22 biogenesis, including maintaining the solubility before membrane insertion, targeting to peroxisomes, and ensuring proper assembly in the peroxisomal boundary membrane.

¹ This work was supported by the Natural Sciences and Engineering Research Council of Canada (grant no. 217291), by the Ontario Premier's Research in Excellence Award (to R.T.M.), and by the Biotechnology and Biology Research Council (grant no. 24/C12039 to A.B.).

^[w] The online version of this article contains Web-only information.

² Present address: Laboratory Services Division, 95 Stone Road West, University of Guelph, Guelph, Ontario, Canada N1H 8J7.

³ Present address: Central Science Laboratory, Sand Hutton, York, UK.

^* Corresponding author; e-mail rtmullen{at}uoguelph.ca; fax 519–767–1991.

Received June 2, 2003; returned for revision June 19, 2003; accepted June 26, 2003.

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