Functional characterization of two missense mutations in Pex5p - C11S and N526K.

[neonatal adrenoleukodystrophy]

Most newly synthesized peroxisomal proteins are targeted to the organelle by Pex 5 p , the peroxisomal cycling receptor . Pex 5 p interacts with these proteins in the cytosol , transports them to the peroxisomal docking /translocation machinery and promotes their translocation across the organelle membrane . Finally , Pex 5 p is recycled back to the cytosol in order to catalyse additional rounds of transportation . Although several properties of this protein sorting pathway have been recently uncovered , we are still far from comprehending many of its basic principles . Here , we describe the mechanistic implications of two single -amino acid substitutions in Pex 5 p . The first mutation characterized , Cys 11 S er , blocks the recycling of Pex 5 p back into the cytosol at the step in which stage 2 Pex 5 p is converted into stage 3 Pex 5 p . The mutation Asn 526 Lys , previously described in a child with neonatal adrenoleukodystrophy and shown to abolish the PTS 1 -binding capacity of Pex 5 p , results in a Pex 5 p protein exhibiting import capacity . Protease assays suggest that the Asn 526 L ys mutation causes conformational alterations at the N-terminal half of Pex 5 p mimicking the ones induced by binding of a PTS 1 -containing peptide to the normal peroxin . The implications of these findings on the mechanism of protein translocation across the peroxisomal membrane are discussed .