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 specifically binds to its receptor SecA. Also required for the secretion of HasA via an ABC transporter system. Homotetramer, a dimer of dimers. One homotetramer interacts with 1 SecA dimer (By similarity). Belongs to the SecB family.