Figure 3.

3D reconstructions of six MBP-inserted Mcm2–7 double hexamers. (A) Two selected reference-free class averages of the double hexamer with MBP inserted to the N-terminal region of Mcm2 (MBP-M2), Mcm3 (MBP-M3), Mcm6 (MBP-M6), and Mcm7 (MBP-M7) and the C-terminal region of Mcm2 (M2-MBP) and Mcm5 (M5-MBP). Blue asterisks mark the extra and peripheral MBP densities. Box size is 36 nm. (B) Each column shows the top, front, and side views of the six MBP-fused Mcm2–7 double hexamers. The MBP density is colored blue in the 3D map. The bottom panels show horizontal sections of the 3D maps at the inserted MBP positions as indicated by dashed lines. Blue asterisks mark the MBP densities in the sections. Box size in the bottom panel is 24 nm. (C) The Mcm subunit arrangement in the lower hexamer as viewed from the N termini (left) and in the upper hexamer viewed from the C termini (right). (D) NTD and CTD assignment of each Mcm subunit in the top hexamer of the double hexamer as viewed from the side. The NTDs and the CTDs are staggered such that each Mcm subunit is tilted in the hexamer, and the double hexamer is twisted. The Mcm protein domains labeled with red numbers in C and D are positively identified in this study, and the domains in blue numbers are inferred from the known order of subunits in the hexamer.