Conformational changes in the bacterial SRP receptor FtsY upon binding of guanine nucleotides and SRP

In cotranslational preprotein targeting in Escherichia coli, the signal rec og- nition particle (SRP) binds to the signal peptide emerging from the rib osome and, subsequently, interacts with the signal recognition particle rec eptor, FtsY, at the plasma membrane. Both FtsY and the protein moiety of th e signal recognition particle, Ffh, are GTPases, and GTP is required for th e formation of the SRP-FtsY complex. We have studied the binding of GTP/GDP to FtsY as well as the SRP-FtsY complex formation by monitoring the fluore scence of tryptophan 343 in the I box of mutant FtsY. Thermodynamic and kin etic parameters of the FtsY complexes with GDP, GTP, and signal recognition particle are reported. Upon SRP-FtsY complex formation in the presence of GTP, the fluorescence of tryptophan 343 increased by 50 % and was blue-shif ted by 10 nm. We conclude that Gm-dependent SRP-FtsY complex formation lead s to an extensive conformational change in the I box insertion in the effec tor region of FtsY. (C) 2000 Academic Press.