One of the proteins required for the normal export of some preproteins out of the cell cytoplasm. It is a molecular chaperone that binds to a subset of precursor proteins, maintaining them in a translocation-competent state. For 2 proteins (MBP, MalE and PhoA) the substrate is wrapped around the homotetramer, which prevents it from folding (PubMed:27501151). It also specifically binds to its receptor SecA. Its substrates include DegP, FhuA, FkpA, GBP, LamB, MalE (MBP), OmpA, OmpF, OmpT, OmpX, OppA, PhoE, TolB, TolC, YbgF, YcgK, YgiW and YncE (PubMed:16352602). Homotetramer, a dimer of dimers (PubMed:14643199, PubMed:27501151). One homotetramer interacts with 1 SecA dimer. Growth continues, but export of its substrates is blocked (PubMed:16352602, PubMed:16522795). Expression of chaperones DnaK, GroEL/ES, ClpB, and HslU increases (PubMed:16352602). Belongs to the SecB family.