(A) Domain structures of wildtype FtsY and FtsY truncation mutants. (B, C) The membrane binding abilities of the truncation mutants on SLB as free FtsY (B) and SRP•FtsY complex (C). Filled and open bars represent populations in the Dynamic and Stable modes, respectively, determined using the lifetime cutoff of 0.25 s. (D) Sequences of the αA1 motif in wildtype and charge mutants. Charged residues in wildtype sequence are highlighted in red, and their mutations are highlighted in blue. (E) Charge mutations in αA1 reduced membrane interactions of free FtsY with SLB. (F, G) Model for regulation of the membrane interactions of full length FtsY (F) and FtsY-NG+1 (G). Cyan and blue arrows denote membrane interactions in the Dynamic and Stable modes, respectively. Both the A-domain (this work) and N-domain (Parlitz et al., 2007; Lam et al., 2010) inhibit FtsY from engaging the membrane in the Stable mode (red inhibition marks). In full-length FtsY, αA1 partially relieves the inhibition from the A-domain (green inhibition mark). In FtsY-NG+1, the αN1 motif can also mediate some degree of Dynamic interactions (dashed arrow in G). With both constructs, interaction with SRP is the most effective mechanism to relieve the inhibitory effect from the N-domain and allow FtsY to interact with the membrane in the Stable mode. Values are reported as mean ± S.D., with n ≥ 3.