The sorting signals for peroxisomal membrane-bound ascorbate peroxidase are within its C-terminal tail

Abstract

Peroxisomal ascorbate peroxidase (APX) is a carboxyl tail-anchored, type II (N(cytosol)-C(matrix)) integral membrane protein that functions in the regeneration of NAD+ in glyoxysomes of germinated oilseeds and protection of peroxisomes in other organisms from toxic H2O2. Recently we showed that cottonseed peroxisomal APX was sorted post-translationally from the cytosol to peroxisomes via a novel reticular/circular membranous network that was interpreted to be a subdomain of the endoplasmic reticulum (ER), named peroxisomal ER (pER). Here we report on the molecular signals responsible for sorting peroxisomal APX. Deletions or site-specific substitutions of certain amino acid residues within the hydrophilic C-terminal-most eight-amino acid residues (includes a positively charged domain found in most peroxisomal integral membrane-destined proteins) abolished sorting of peroxisomal APX to peroxisomes via pER. However, the C-terminal tail was not sufficient for sorting chloramphenicol acetyltransferase to peroxisomes via pER, whereas the peptide plus most of the immediately adjacent 21-amino acid transmembrane domain (TMD) of peroxisomal APX was sufficient for sorting. Replacement of the peroxisomal APX TMD with an artificial TMD (devoid of putative sorting sequences) plus the peroxisomal APX C-terminal tail also sorted chloramphenicol acetyltransferase to peroxisomes via pER, indicating that the peroxisomal APX TMD does not possess essential sorting information. Instead, the TMD appears to confer the proper context required for the conserved positively charged domain to function within peroxisomal APX as an overlapping pER sorting signal and a membrane peroxisome targeting signal type 2.