Comparison of structures containing the BTB fold. (a) Superposition of the BTB core fold from currently known BTB structures. The BTB core fold (approximately 95 residues) is retained across four sequence families. The BTB-ZF, Skp1, ElonginC and T1 families are represented here by the domains from Protein Data Bank (PDB) structures 1buo:A, 1fqv:B, 1vcb:B, 1t1d:A. (b) Schematic of the BTB fold topology. The core elements of the BTB fold are labeled B1 to B3 for the three conserved β-strands, and A1 to A5 for the five α-helices. Many families of BTB proteins are of the 'long form', with an amino-terminal extension of α1 and β1. Skp1 proteins have two additional α-helices at the carboxyl terminus, labeled α7 and α8. The dashed line represents a segment of variable length that is often observed as strand β5 in the long form of the domain, and as an α-helix in Skp1. (c) Structure-based multiple sequence alignment of representative BTB domains from each of the BTB-ZF, Skp1, ElonginC and T1 families. The core BTB fold is boxed. Secondary structure is indicated by red shading for α-helices and yellow for β-strands, with the amino- and carboxy-terminal extensions shaded in gray. The low complexity sequences, which are disordered in the Skp1 structures, are indicated by open triangles. See Figure 3 for the PDB codes for the corresponding sequences.