FIGURE 3.

Comparison of tetrameric interfaces with a previously validated trimeric interface. A, schematic representations of a trimeric interface previously validated by cross-linking (35) and two previously proposed tetrameric configurations from functional studies of covalently linked concatemers (16). Cross-linking data from Collier et al. (41) suggest a trimeric clockwise topology of αγβ (35). These data also agree with two coexisting functional tetrameric configurations. Intersubunit angles vary slightly between trimer and tetramer models, resulting in slightly different interfaces as follows: in clockwise order, αβ, βγ, and γα, in addition to two unique interfaces to tetramers α₁α₂ and α₁γ. B, schematic representation of α₂β and α₂γ interfaces in the tetramer model α₁βα₂γ. Shown as sticks, βV85 can cross-link with αLys-477, and γGlu-455 can cross-link with αLeu-120. Cross-linkers are represented as red bars.