3D Complex: A Structural Classification of Protein Complexes
Figure 6
Cyclic and Dihedral Symmetries
(C2) Cyclic symmetry: two subunits are related by a single 2-fold axis, shown by a dashed line. An ellipse at the end of the symmetry axis marks a 2-fold axis. Nearly all homodimers have C2 symmetry. C2 symmetry is termed “2” in the crystallographic Hermann-Mauguin nomenclature, shown in red beneath C2.
(C4) Cyclic symmetry: four subunits are related by one 4-fold axis. A square at the end of the symmetry axis marks a 4-fold axis.
(D2) Dihedral symmetry: four subunits are related by three 2-fold axes. D2 symmetry can be constructed from two C2 dimers. Note the difference between the D2 and C4 symmetries: two symmetry types that both have four subunits.
(D4) Dihedral symmetry: eight subunits are related to each other by one 4-fold axis and two 2-fold axes. Note that D4 symmetry can be constructed by stacking two C4 tetramers as shown, or four C2 dimers (not shown).