AtPEX2 and AtPEX10 are targeted to peroxisomes independently of known endoplasmic reticulum trafficking routes

Controversy exists in the literature over the involvement of the endoplasmic reticulum (ER) in the delivery of membrane proteins to peroxisomes. In this study, the involvement of the ER in the trafficking of two Arabidopsis (Arabidopsis thaliana) peroxisomal membrane proteins was investigated using confocal laser scanning microscopy of living cells expressing fusions between enhanced yellow fluorescent protein (eYFP) and AtPEX2 and AtPEX10. The fusion proteins were always detected in peroxisomes and cytosol irrespective of the location of the eYFP tag or the level of expression. The cytosolic fluorescence was not due to cleavage of the eYFP reporter from the C-terminal fusion proteins. Blocking known ER transport routes using the fungal metabolite Brefeldin A or expressing dominant negative mutants of Sar1 or RabD2a had no effect on the trafficking of AtPEX2 and AtPEX10 to peroxisomes. We conclude that AtPEX2 and AtPEX10 are inserted into peroxisome membranes directly from the cytosol.