Abstract
FtsY and Ffh are structurally similar prokaryotic Signal Recognition Particle GTPases that play an essential role in the SRP-mediated co-translational targeting of proteins to the membrane. The two GTPases assemble in a GTP-dependent manner to form a heterodimeric SRP targeting complex. We report here the 2.1Å X-ray structure of FtsY from T. aquaticus bound to GDP. The structure of the monomeric protein reveals, unexpectedly, canonical binding interactions for GDP. A comparison of the structures of the monomeric and complexed FtsY NG GTPase domain suggests that it undergoes a conformational change similar to that of Ffh NG during the assembly of the symmetric heterodimeric complex. However, in contrast to Ffh, in which the C-terminal helix shifts independently of the other sub-domains, the C-terminal helix and N domain of T. aquaticus FtsY together behave as a rigid body during assembly, suggesting distinct mechanisms by which the interactions of the NG domain 'module' are regulated in the context of the two SRP GTPases.
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