Pnc1 piggy-back import into peroxisomes relies on Gpd1 homodimerisation

Nadal A Al Saryi,Ewald H Hettema,Patrick Baker,John D Hutchinson,Murtakab Y Al-Hejjaj,Svetlana Sedelnikova

doi:10.1038/srep42579

Abstract

Peroxisomes are eukaryotic organelles that posttranslationally import proteins via one of two conserved peroxisomal targeting signal (PTS1 or 2) mediated pathways. Oligomeric proteins can be imported via these pathways but evidence is accumulating that at least some PTS1-containing monomers enter peroxisomes before they assemble into oligomers. Some proteins lacking a PTS are imported by piggy-backing onto PTS-containing proteins. One of these proteins is the nicotinamidase Pnc1, that is co-imported with the PTS2-containing enzyme Glycerol-3-phosphate dehydrogenase 1, Gpd1. Here we show that Pnc1 co-import requires Gpd1 to form homodimers. A mutation that interferes with Gpd1 homodimerisation does not prevent Gpd1 import but prevents Pnc1 co-import. A suppressor mutation that restores Gpd1 homodimerisation also restores Pnc1 co-import. In line with this, Pnc1 interacts with Gpd1 in vivo only when Gpd1 can form dimers. Redirection of Gpd1 from the PTS2 import pathway to the PTS1 import pathway supports Gpd1 monomer import but not Gpd1 homodimer import and Pnc1 co-import. Our results support a model whereby Gpd1 may be imported as a monomer or a dimer but only the Gpd1 dimer facilitates co-transport of Pnc1 into peroxisomes.

Highlights

Peroxisomes are organelles that participate in a large number of cellular processes including fatty acid oxidation and other hydrogen peroxide-producing oxidation reactions
We find that Gpd[1] can be imported as a dimer via the PTS2 pathway and that a mutant that is affected in dimer formation is still imported
Our study supports a model whereby Gpd[1] forms dimers in the cytosol to which Pnc[1] binds before they are co-imported via the PTS2 import pathway

Summary

Introduction

Peroxisomes are organelles that participate in a large number of cellular processes including fatty acid oxidation and other hydrogen peroxide-producing oxidation reactions. Import of oligomers has been widely reported (for review see refs 11, 28 and 29), oligomers may not be the preferred substrate for the PTS1 import route as for several abundant peroxisomal proteins it was shown that Pex[5] binding prevents oligomerisation in vitro[30,31] This suggests that Pex[5] is both acting as a targeting receptor and a molecular chaperone that regulates oligomerisation[29]. The two examples of these, superoxide dismutase and lactate dehydrogenase both display partially peroxisomal localisation and cytosolic localisation[32,33] Another example of hetero-oligomeric import is Pyrazinamidase/Nictonamidase I (Pnc1), which is targetted via the PTS2. During the course of our study, two independent groups reported that Pnc[1] is co-imported with the PTS2-containing protein Gpd[1] (glycerol 3-phosphate dehydrogenase 1)[37,38]

Methods

Results

Conclusion