Peroxisomal proteins carrying a type 1 peroxisomal targeting signal (PTS1) are recognized by the well-conserved cycling import receptor Pex5p. Yeast YMR018w gene codes for a Pex5p paralog and novel peroxin that is involved in peroxisomal import of a subset of matrix proteins. The novel peroxin was designated Pex9p and it interacts with the docking protein Pex14p and a subclass of PTS1-containing peroxisomal matrix enzymes. Unlike Pex5p, Pex9p is not expressed in glucose- or ethanol-grown cells, but it is strongly induced by oleate. Under these conditions, Pex9p acts as a cytosolic and membrane-bound peroxisome import receptor for both malate synthase isoenzymes, Mls1p and Mls2p. The inducible Pex9p-dependent import pathway provides a rationale for the oleate-inducible peroxisomal targeting of malate synthases. The existence of two distinct PTS1-receptors, in addition to two PTS2-dependent import routes, contributes to the adaptive metabolic capacity of peroxisomes in response to environmental changes and underlines the role of peroxisomes as multi-purpose organelles. The discovered differential import into peroxisomes contributes to the molecular understanding of how regulated protein targeting can adapt the function of organelles according to cellular needs.

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