Figure 2. 3mbt Adopts a Triangular Propeller-like Architecture.

(A) Ribbon representation (orthorhombic crystal) of the structure (β subunit core face) of 3mbt. The first (residues 204–309), second (residues 310–416), and third (residues 417–527) mbt repeats are colored cyan, red, and yellow, respectively. Secondary structure elements are indicated for the first mbt repeat, which comprises a long N-terminal arm (αB, αC, and αD) and a β subunit core (β1, β2, β3, β1, β4, β5, α2, and β6). αA exists only in the second and third repeats and is indicated explicitly for clarity. α1 of the second repeat is shown to clarify the unique kissing α1–β 2 interaction (see text and Figure 4C).

(B) Electrostatic surface representation (orthorhombic crystal) of the β subunit core face of the triangular propeller-like 3mbt structure. Blue, electropositive; red, electronegative. This face, with its extensive acidic (red) patches, contains three ligand binding pockets (numbers 1, 2, and 3 refer to the pockets belonging to the first, second, and third repeats, respectively).

(C) Electrostatic surface representation (orthorhombic crystal) of the opposite bowl-like face of 3mbt. This face, which contains basic (blue) patches on the periphery, is comprised of N-terminal arms and predominantly exhibits nonpolar character. Some electropositive charge is clustered toward one side of the surface.

A central cavity with approximate dimensions of 10 Å × 15 Å can be clearly seen in both (B) and (C).