Functional analysis of PEX13 mutation in a Zellweger syndrome spectrum patient reveals novel homooligomerization of PEX13 and its role in human peroxisome biogenesis

In humans, the concerted action of at least 13 different peroxisomal PEX proteins is needed for proper peroxisome biogenesis. Mutations in any of these PEX genes can lead to lethal neurometabolic disorders of the Zellweger syndrome spectrum (ZSS). Previously, we identified the W313G mutation located within the SH3 domain of the peroxisomal protein, PEX13. As this tryptophan residue is highly conserved in almost all known SH3 proteins, we investigated the pathogenic mechanism of the W313G mutation and its role in PEX13 interactions and functions in peroxisome biogenesis. Here, we report for the first time that human PEX13 interacts with itself in peroxisomes in living cells. We demonstrate that the import of PTS1 (peroxisomal targeting signal 1) proteins is specifically disrupted when homooligomerization of PEX13 is interrupted. Live cell FRET microscopy in living cells as well as co-immunoprecipitation experiments reveal that the highly conserved W313 residue is important for self-association of PEX13 but is not required for interaction with PEX14, a well-established interaction partner at the peroxisomal membrane. Experiments with truncated constructs indicate that although the W313G mutation resides in the C-terminal SH3 domain, the N-terminal half is necessary for peroxisomal localization, which in turn appears to be crucial for homooligomerization. Furthermore, rescue of homooligomerization in the W313G mutant cells through complementation with truncation constructs restores import of peroxisomal matrix proteins. Taken together, the thorough analyses of a ZSS patient mutation unraveled the general cell biological function of PEX13 and its mechanism in the import of peroxisomal matrix PTS1 proteins.