Unique double-ring structure of the peroxisomal Pex1/Pex6 ATPase complex revealed by cryo-electron microscopy

Significance Pex1 and Pex6 are members of the AAA family of ATPases, which contain two ATPase domains in a single polypeptide chain and form hexameric double rings. These two Pex proteins are involved in the biogenesis of peroxisomes, and mutations in them frequently cause diseases. Here, we determined structures of the Pex1/Pex6 complex by cryo-electron microscopy. Novel computational modeling methods allowed placement of Pex1/Pex6 domains into subnanometer density maps. Our results show that the peroxisomal Pex1/Pex6 ATPases form a unique double-ring structure in which the two proteins alternate around the ring. Our data shed light on the mechanism and function of this ATPase and suggest a role in peroxisomal protein import similar to that of p97 in ER-associated protein degradation. Members of the AAA family of ATPases assemble into hexameric double rings and perform vital functions, yet their molecular mechanisms remain poorly understood. Here, we report structures of the Pex1/Pex6 complex; mutations in these proteins frequently cause peroxisomal diseases. The structures were determined in the presence of different nucleotides by cryo-electron microscopy. Models were generated using a computational approach that combines Monte Carlo placement of structurally homologous domains into density maps with energy minimization and refinement protocols. Pex1 and Pex6 alternate in an unprecedented hexameric double ring. Each protein has two N-terminal domains, N1 and N2, structurally related to the single N domains in p97 and N-ethylmaleimide sensitive factor (NSF); N1 of Pex1 is mobile, but the others are packed against the double ring. The N-terminal ATPase domains are inactive, forming a symmetric D1 ring, whereas the C-terminal domains are active, likely in different nucleotide states, and form an asymmetric D2 ring. These results suggest how subunit activity is coordinated and indicate striking similarities between Pex1/Pex6 and p97, supporting the hypothesis that the Pex1/Pex6 complex has a role in peroxisomal protein import analogous to p97 in ER-associated protein degradation.