Recent studies have demonstrated that bacteria possess an essential protein translocation system similar to mammalian signal recognition particle (SRP). Here we have identified the Ffh, a homologue of the mammalian SRP54 subunit from S. pneumoniae. Ffh is a 58-kDa protein with three distinct domains: an N-terminal hydrophilic domain (N-domain), a G-domain containing GTP/GDP binding motifs, and a C-terminal methionine-rich domain (M-domain). The full-length Ffh and a truncated protein containing N and G domains (Ffh-NG) were overexpressed in E. coli an purified to homogeneity. The full-length Ffh has an intrinsic GTPase activity with k(cat) of 0.144 min(-1), and the K-m for GTP is 10.9 muM. It is able to bind to 4.5S RNA specifically as demonstrated by gel retardation assay. The truncated Ffh-NG has approximately the same intrinsic GTPase activity to the full-length Ffh, but is unable to bind to 4.5S RNA, indicating that the NG domain is sufficient for supporting intrinsic GTP hydrolysis, and that the M domain is required for RNA binding. The interaction of S. pneumoniae Ffh with its receptor, FtsY, resulted in a 20-fold stimulation in GTP hydrolysis. The stimulation was further demonstrated to be independent of the 4.5S RNA. In addition, a similar GTPase stimulation is also observed between Ffh-NG and FtsY, suggesting that the NG domain is sufficient and the M domain is not required for GTPase stimulation between Ffh and FtsY. (C) 2002 Elsevier Science (USA).