(A) In the Rea1 AAA+ ring, the H3 α-helices of AAA1L, AAA2L, AAA3L, AAA5L and AAA6L (red) point towards the pore of the AAA+ ring. The orientation of AAA4L (yellow) deviates from the orientation of the other AAA+ domains in the ring. It is rotated towards AAA5L as indicated by its H3 α-helix that points towards AAA5L rather than the AAA+ ring centre. For clarity only the AAAL domains of the AAA+ ring are shown and all inserts have been removed. (B) Comparison of Rea1 AAA+ ring with the ring of a dynein crystal structure in the ADP state. For clarity, only the AAAL domains are shown. Left panel: In Rea1, the AAA1L, AAA2L and AAA6L domains are more tightly associated and form one half of the AAA+ring. AAA3L, AAA4L, and AAA5L form the more loosely associated second half. The AAA1L-AAA2L + AAA6L domain block is separated from the AAA3L-AAA5L block by large gaps between AAA2L and AAA3L as well as AAA6L and AAA5L (black arrow heads). The internal organisation of the AAA3L-AAA5L half is characterised by the gap between AAA3L and AAA4L (grey arrow head), as well as the rotation of AAA4L towards AAA5L (both domains are shown in grey, with red H3 helices to indicate their relative orientation with respect to each other). Right panel: The dynein AAA+ ring (PDB-ID: 3VKG) is highly similar. Here, AAA2L-AAA4L form the more tightly packed first half of the ring. The weaker packed second half consists of AAA5L-AAA6L + AAA1L. Large gaps (black arrow heads) separate the two halves. The internal organisation of the second half is characterised by a gap between AAA5L and AAA6L (grey arrow head) and the rotation of AAA6L towards AAA1L (both highlighted in grey with H3 helices in red to indicate the orientation of AAA6L towards AAA1L).