The 54 kD subunit of the signal recognition particle (SRP54) is a multi-domain protein which binds 7S RNA, binds to and hydrolyzes GTP and recognizes signal sequences as they emerge from the translating ribosome. We seek to understand the mechanism by which SRP54 coordinates binding and release of signal sequences during targeting of the nascent chain to the translocation apparatus of the cell. Therefore, we initiated structural studies of the SRP54 homologue from Thermus aquaticus (termed Ffh).