Volume 2, Issue 7, Pages 571-573 (July 1994) Proteins with a ring  Joël Janin  Structure  Volume 2, Issue 7, Pages 571-573 (July 1994) DOI: 10.1016/S0969-2126(00)00058-7

Figure 1 Porcine ribonuclease inhibitor. The horseshoe-like structure is made up of a 17-stranded parallel β-sheet (blue arrows) and 16 connecting α-helices (red coils). The ring has an outer diameter of 70 å , an inner diameter of about 20 å ; the open space between the amino-  and carboxy-terminal β-strands is 13 å wide. The 98 leucines and 30 cysteines (out of 456 residues) are shown in ball-and-stick representation. The cysteines have a free SH group (yellow spheres) in the active form of the protein. They undergo oxidation by an all-or-none mechanism [16] that must require a major conformational change, since no disulphide bonds can be formed with the SH groups placed the way they are. [This drawing has been prepared with MOLSCRIPT [17] and D Bacon's Raster3D program using atomic coordinates kindly provided by B Kobe and J Deisenhofer (University of Texas Southwestern Medical Center, Dallas).] Structure 1994 2, 571-573DOI: (10.1016/S0969-2126(00)00058-7)

Figure 2 E. coli soluble lytic transglycosylase. (a) Residues 1– 360, in blue, form 22 α-helices arranged in a right-handed superhelix with a kink in the middle near helix H12. This amino-terminal domain is termed the U domain because of its U-shaped conformation. Residues 361–450, in green, form a linker domain with 5 α-helices, and residues 451–618, in yellow, form the catalytic domain that resembles phage T4 lysozyme. (b) The catalytic domain has been removed to show the 80 å diameter ring with a 25–30 å hole formed by the 27 helices. [Figures kindly provided by BW Dijkstra (University of Groningen).] Structure 1994 2, 571-573DOI: (10.1016/S0969-2126(00)00058-7)

Figure 3 E. coli DNA topoisomerase I. (a) The 590-residue amino-terminal fragment folds into four domains with a total of 18 α-helices (A–R) and 14 β-strands (1–14). Domain I contains a Rossmann fold. The ring, approximately 70 å in diameter, is formed by the two large β-sheets in domain II and by helical domains III and IV. (b) The hole in the ring (shown here face-on and from one side) is large enough (27 å) for double-stranded DNA to go through. [Figures kindly provided by A Mondragón (Northwestern University, Illinois).] Structure 1994 2, 571-573DOI: (10.1016/S0969-2126(00)00058-7)

Figure 3 E. coli DNA topoisomerase I. (a) The 590-residue amino-terminal fragment folds into four domains with a total of 18 α-helices (A–R) and 14 β-strands (1–14). Domain I contains a Rossmann fold. The ring, approximately 70 å in diameter, is formed by the two large β-sheets in domain II and by helical domains III and IV. (b) The hole in the ring (shown here face-on and from one side) is large enough (27 å) for double-stranded DNA to go through. [Figures kindly provided by A Mondragón (Northwestern University, Illinois).] Structure 1994 2, 571-573DOI: (10.1016/S0969-2126(00)00058-7)