Figure 2.

The structures of three RNase molecules, with red domains all in the same orientation. The hinge loops (residues 16–22) are shown in green. (A) The RNase A monomer (Protein Data Bank code: 1RTB) (28). The helix to be swapped (residues 1–15) is shown in blue; the major domain (residues 23–124) is shown in red. The “closed interface” is that between the blue helix and the red major domain. (B) The BS-RNase domain-swapped dimer (Protein Data Bank code: 1BSR) (2). The two intersubunit disulfide bonds are shown in yellow. The “open interface” here is between the adjacent red and blue helices. (C) The RNase A dimer. Subunit 1 (blue, residues 1–124) and subunit 2 (red, residues 201–324) are related by a ∼160-degree rotation about an axis roughly perpendicular to the page. The hinge loops are in the same orientation as those in Fig. 1. Notice the 6-stranded beta-sheet formed from three strands of each subunit. The open interface here is between the adjacent red and blue strands of the beta-sheet. Notice also that the open interfaces in B and C differ but that the closed interfaces in A, B, and C are the same. Functional unit 1 (see Table 3) consists of the blue swapped helix and the red major domain. The diagrams are made with the program molscript (37).