Figure 6. The switch mechanism is shared with proteasome proteases.
A is a secondary structure topology diagram for SpoIIE (left) and for HslV (the E. coli homolog of the proteasome protease; right). β strands are shown as arrows pointing from N to C terminus and α-helices as circles in cross section. Conserved features are dark grey, whereas variable features are light grey. The conserved glycine that moves to activate each protein is indicated with a red circle. The switch helices of SpoIIE and the corresponding α-helices of HslV are colored orange. B shows ribbon diagrams of SpoIIE and HslV (PDB ID 1G3I) colored as in A. The position of the conserved regulatory glycine (G649 in SpoIIE, and G69 in HslV) is shown with a red sphere and the insertions specific to each protein are indicated by brackets. C is a schematic of how the regulatory particle (blue) activates the proteasome proteases (grey). D shows an overlay of the active (PDB ID 1G3I) and inactive (PDB ID 1G3K) states of HslV following superimposition of the regions in grey. The switch helices are color-coded orange and light orange for the active and inactive states, respectively. The active site residues T1, K33, and the carbonyl oxygen of G69 are shown.