SecA is an essential multifunctional protein for the translocation of proteins across bacterial membranes. Though SecA is known to function in the membrane, the detailed mechanism for this process remains unclear. In this study we constructed a series of SecA N-terminal deletions and identified two specific domains crucial for initial SecA/membrane interactions. The first small helix, the linker and part of the second helix (Δ2–22) were found to be dispensable for SecA activity in complementing the growth of a SecA ts mutant. However, deletions of N-terminal aminoacyl residues 23–25 resulted in severe progressive retardation of growth. Moreover, a decrease of SecA activity caused by N-terminal deletions correlated to the loss of SecA membrane binding, formation of lipid-specific domains and channel activity. All together, the results indicate that the N-terminal aminoacyl residues 23–25 play a critical role for SecA binding to membranes and that the N-terminal limit of SecA for activity is at the 25th amino acid.