The plant PTS1 receptor: similarities and differences to its human and yeast counterparts

Two targeting signals, PTS1 and PTS2, mediate import of proteins into the p eroxisomal matrix. We have cloned and sequenced the watermelon (Citrullus v ulgaris) cDNA homologue to the PTS1 receptor gene (PEX5). Its gene product, CvPex5p, belongs to the family of tetratricopeptide repeat (TPR) containin g proteins like the human and yeast counterparts, and exhibits 11 repeats o f the sequence W-X-2-(E/S)-(Y/F/Q) in its N-terminal half. According to fra ctionation studies the plant Pex5p is located mainly in the cytosolic fract ion and therefore could function as a cycling receptor between the cytosol and glyoxysomes, as has been proposed for the Pex5p of human and some yeast peroxisomes. Transformation of the Hansenula polymorpha peroxisome deficie nt pex5 mutant with watermelon PEX5 resulted in restoration of peroxisome f ormation and the synthesis of additional membranes surrounding the peroxiso mes. These structures are labeled in immunogold experiments using antibodie s against the Hansenula polymorpha integral membrane protein Pex3p, confirm ing their peroxisomal nature. The plant Pex5p was localized by immunogold l abelling mainly in the cytosol of the yeast, but also inside the newly form ed peroxisomes. However, import of the PTS1 protein alcohol oxidase is only partially restored by CvPex5p.