SecA

This amino acid domain is subdivided into the two nucleotide binding folds 1 and 2 (NBF1 and NBF2) where ATP is bound and hydrolyzed. The chemical energy from the phosphodiester bonds results in a conformational change which is transferred to other domains (especially the HWD and the PPXD domains) which consequently mechanically move the preprotein across the membrane. However, these conformational changes are partly regulated by other protomer domains described below.

The capability to bind to the SecB chaperone during post-translational translocation, the ribosome (during both post-translational translocation and co-translational translocation [3]) and the phospholipid bilayer is important for SecA functioning and is achieved by the C-terminal linker domain.[4]

Located at the C-terminal portion of the molecule, this domain is in contact with the HSD and PPXD domains. Likely it plays a role in transferring molecular conformational motion, which it receives from HSD and which originates from ATP hydrolysis in the DEAD motor domain, to the PPXD domain.

Since SecA's essential function is the transport of preprotein across the membrane the ability to actually bind preprotein must be given. The PPXD domain fulfils this function upon substrate binding.

This domain lies in the center of the SecA protomer and contacts via α-helical interactions all other subdomains. In addition it contains the intramolecular regulator of ATP hydrolysis 1 (IRA1) subdomain which seems to prevent unwanted ATP hydrolysis when SecA is not bound to SecYEG. Together with IRA1, a conserved salt bridge called Gate 1 might function to prevent unnecessary conformational change. Gate 1 seems to functionally connect the nucleotide (ATP) binding site of the DEAD motor domain with the PPXD domain which results in regulation of ATP hydrolysis only upon preprotein binding. However, this coordinative behaviour has only been shown to occur when SecA is bound to SecYEG.[5]