DIFFERENTIAL PROTEIN IMPORT DEFICIENCIES IN HUMAN PEROXISOME ASSEMBLYDISORDERS

Two peroxisome targeting signals (PTSs) for matrix proteins have been well defined to date. PTS1 comprises a COOH-terminal tripeptide, SKL, and has been found in several matrix proteins, whereas PTS2 has been f ound only in peroxisomal thiolase and is contained within an NH2-termi nal cleavable presequence. We have investigated the functional integri ty of the import routes for PTS1 and PTS2 in fibroblasts from patients suffering from peroxisome assembly disorders. Three of the five compl ementation groups tested showed a general loss of PTSI and PTS2 import . Two complementation groups showed a differential loss of peroxisomal protein import: group I cells were able to import a PTS1- but not a P TS2-containing reporter protein into their peroxisomes, and group IV c ells were able to import the PTS2 but not the PTS1 reporter into aberr ant, peroxisomal ghostlike structures. The observation that the PTS2 i mport pathway is intact only in group IV cells is supported by the pro tection of endogenous thiolase from protease degradation in group TV c ells and its sensitivity in the remaining complementation groups, incl uding the partialized disorder of group I. The functionality of the PT S2 import pathway and colocalization of endogenous thiolase with the p eroxisomal membranes in group IV cells was substantiated further using immunofluorescence, subcellular fractionation, and immunoelectron mic roscopy. The phenotypes of group I and IV cells provide the first evid ence for differential import deficiencies in higher eukaryotes. These phenotypes are analogous to those found in Saccharomyces cerevisiae pe roxisome assembly mutants.