Structure of the AAA+ module. (a) Monomeric AAA+ module of Aquifex aeolicus DnaA, a protein involved in the initiation of DNA replication (Protein Data Bank (PDB) code 2HCB) . The α-helices and random coils are in green and the β-strands of the core αβα nucleotide-binding domain are in blue, with the exception of the two equal-sized helical inserts, which are colored pink. The small α-helical domain is colored purple. (b) Major motifs in the AAA+ module of (a) are colored as indicated in the key, on the basis of the alignment in reference . The bound adenosine 5'-[β,γ-methylene]triphosphate (β,γ-methylene-ATP, a nonhydrolyzable ATP analog, orange sticks) and Mg2+ (black sphere) are also shown. (c) Top and side views of the hexameric structure of Haemophilus influenzae HslU, a member of the HslU/ClpX family (PDB 1KYI) . α-Helices, including random coils, and β-strands of the core αβα nucleotide-binding domain are colored green and blue, respectively. Two additional helices characteristic of HslU-family proteins, called the I domain, are colored orange, and an additional extended loop between the second core β-strand and the following helix is colored in pink. The core small α-helical domain is colored purple, with the two-stranded β-sheet insertion in yellow. Structures were drawn using PyMOL .