Allosteric regulation of a prokaryotic small Ras-like GTPase contributes to cell polarity oscillations in bacterial motility
Fig 2
Crystal structures of M. xanthus MglA and MglAB–GTPγS complex.
(A) Crystal structure of MxMglA bound to GDP. The secondary structure elements are labeled. GDP is shown in stick representation in light brown, and the catalytic loops switch 1 (blue) and switch 2 (red) are labeled as SW1 and SW2. The rest of the loops are shown in light pink, whereas the secondary structure elements are shown in shades of green. The C-terminal hexa-histidine tag (His)6, which is ordered in the crystal structure, is shown in gray. (B) Crystal structure of the complex of MxMglA (green) bound to GTPγS (yellow) and MxMglB (magenta). “Front” view and a 90° rotated view (side view) are shown. The 2 protomers of MxMglB are labeled as MglB1 and MglB2. The C-terminal helix of MxMglB (Ct-helix), the respective N- and C-terminal ends, switch 1 (blue; SW1) and switch 2 (red; SW2), and relevant secondary structures of MxMglA are labeled. GTPγS is shown in stick representation in yellow. Dotted magenta line connects the ends on either sides of the stretch of disordered residues in MglB1. Boxed regions in the front and side view panels highlight the major interfaces between MxMglA with the Rbl/LC7 domain of MxMglB (box with short dashes) and the Ct-helix of MxMglB (box with long dashes), respectively. (C) Residues of β2 strand (Phe56A, Phe57A, and Phe59A) of MxMglA (green) form a hydrophobic surface that interacts with MxMglB (magenta). The relevant secondary structure elements are labeled, and switch 1 and 2 (SW1 and SW2) are colored in blue and red, respectively. The region corresponds to a zoomed view of the boxed region (short dashes) in the front view of panel B. (D) The interacting interface of the Ct-helix of MxMglB (magenta) and MxMglA (green). The side chains of relevant interface residues are shown in stick representation and labeled. Water molecules are represented as red spheres, and dotted yellow lines represent hydrogen bond interactions. The region corresponds to a zoomed view of the boxed region (long dashes) in the side view of panel B. (E and F) Conformational changes in switch 1 (SW1; 2 shades of blue) and switch 2 (SW2; 2 shades of red) in the presence of MxMglB illustrates the mechanism of GAP activity (by reorientation of active site residues Arg53A and Glu82A). MxMglA–GDP conformation and MglA from the MxMglAB–GTPγS complex are shown in panels E and F, respectively. The 2 structures from a superposed orientation are shown as separate panels. Secondary structures extending from the switch loops for MxMglA–GDP conformation and MglA from the MxMglAB–GTPγS complex are shown in brown and green, respectively. Switch 1 (or 2) of the 2 structures are labeled SW1 (or 2) and SW1 (or 2) using GDP and GSP as subscripts for the MxMglA–GDP and MxMglAB–GTPγS complexes, respectively. (G and H) Ct-helix of MxMglB (magenta) and GTP (represented by GTPγS shown in stick representation) bind to the C- and N-terminal ends of the β2 strand of MxMglA, respectively. MxMglA–GDP conformation and MglA from the MxMglAB–GTPγS complex are shown in panels G and H, respectively. The 2 structures from a superposed orientation are shown as separate panels. The unflipped and flipped states of the strand are shown (brown, MxMglA–GDP; green, MglA conformation in MxMglAB–GTPγS structure; switch 1 and switch 2 are in shades of blue and red, respectively). A schematic representation of the registry shift and β2 flip is shown below, with the same color scheme. A 6-pointed yellow star and a 5-pointed light brown star represent GTP (labeled as “T’) and GDP (labeled as “D”), respectively. The β2 strand is represented by an arrow. The residues that form the interface are schematically shown by 2 circles on the bottom or top of the β2 strand representation in the flipped and unflipped states of the strand, respectively. Ct-helix of MxMglB is schematically shown by a cylinder that is labeled “H.” Ct-helix, C-terminal helix; GAP, GTPase activating protein; GTPγS, guanosine 5’-O-[gamma-thio]triphosphate; Mgl, mutual gliding motility; MxMglA, M. xanthus MglA; MxMglB, M. xanthus MglB; Rbl, Roadblock; SW1, switch 1; SW2, switch 2.