One of the proteins required for the normal export of 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. It also specifically binds to its receptor SecA. Homotetramer, a dimer of dimers. One homotetramer interacts with 1 SecA dimer. Belongs to the SecB family.