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Figure 1.
Fig. 1. Structure of FlhF in comparison with the
SRP-targeting complex. (a) Domain structure of the SRP GTPases
FlhF, SRP54, and FtsY. The positions of SRP GTPase-specific
motifs, the I-box, and the conserved nucleotide-binding elements
(G1–G5) are indicated. (b) Ribbon representation of the FlhF
homodimer (green, Left) with two GTP molecules viewed
perpendicular to the 2-fold axis (dashed line) and the SRP/SR
heterodimer from T. aquaticus (blue, Right) with two GMPPCP
molecules (21). The N domains of the two complexes are on top
and the G domains at the bottom. Three motifs involved in domain
communication in the SRP-targeting complex are in yellow. In
FlhF these motifs are absent. In the FlhF homodimer, the N
domains are not part of the dimer interface (monomers are
labeled FlhF and FlhF') and are separated by 12 Å. The G
domains of FlhF and FlhF' form a composite active site harboring
two nucleotides similar to the SRP/SR heterodimer. (c) Sequence
alignment of regulatory motifs in SRP GTPases: "ALLEADV,"
"DARGG," and "GQ." FlhF from B. subtilis is compared with SRP54
(Ffh) and FtsY from S. solfataricus (Sol), T. aquaticus (Taq)
and E. coli (Ec). (d) Close-up of the N/G interdomain region in
the FlhF homodimer shows the position of the tyrosine insertion.
In Figs. 1 Go- –3, SI Figs. 4
and 6, and SI Table 3, the T. aquaticus structure (21) is used
as example for the SRP/SR heterodimer. The S. solfataricus
structure (G.B. and I.S., unpublished data) gives very similar
results.
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