Ribose-binding protein (RBP) is an exported protein of Escherichia coli that functions in the periplasm. The export of RBP involves the secretion machinery of the cell, consisting of a cytoplasmic protein, SecA, and the integral membrane translocation complex, including SecE and SecY. SecB protein, a chaperone known to mediate the export of some periplasmic and outer membrane proteins, was previously reported not to be involved in RBP translocation even though small amounts of in vitro complexes between SecB and RBP have been detected. In our investigation, it was shown that a dependence on SecB could be demonstrated under conditions in which export was compromised. Species of RBP which carry two mutations, one in the leader that blocks export and a second in the mature protein which partially suppresses the export defect, were shown to be affected by SecB for efficient translocation. Five different changes which suppress the effect of the signal sequence mutation -17LP are all located in the N domain of the tertiary structure of RBP. All species of RBP show similar interaction with SecB. Furthermore, a leaky mutation, -14AE, generated by site-specific mutagenesis causes reduced export in the absence of SecB. These results indicate that SecB can interact with RBP during secretion, although it is not absolutely required under normal circumstances.The ribose-binding protein (RBP) of Escherichia coli functions in the periplasm as a component of a high-affinity transport system for ribose (38) and as a primary receptor for bacterial chemotaxis toward ribose (1). RBP is exported by components involved in the translocation of other secreted proteins, such as maltose-binding protein (MBP), bacteriophage lambda receptor (LamB), and other periplasmic and outer membrane proteins. However, it has been shown that the translocation of RBP synthesized in the cytoplasm is independent of translocation of the export component SecB (5, 13, 15), although some interaction between SecB and RBP has been demonstrated in vitro (8,12,14).SecB function was originally identified by a genetic defect in protein secretion. The protein exists as an oligomer with a monomeric subunit of 16.6 kDa (36), which appears to be required for the export of a subset of proteins that are secreted via a SecA-and SecY (PrlA)-dependent pathway (9). The periplasmic protein MBP and outer membrane proteins LamB, OmpA, and OmpF are SecB dependent, whereas ,-lactamase, lipoprotein, phage M13 coat protein, and RBP are SecB independent (13). SecB has been proposed to function as a molecular chaperone that maintains protein folding intermediates, perhaps structurally similar to the molten globule (21), by preventing the protein from attaining its final conformation (4). Other chaperones, for example, GroEL and Hsp7O, are also known to be involved in the export of ,B-lactamase (3) and in the import of protein into the mitochondria (7), respectively. However, the crossspecificity of these proteins for their target substrates with respect to protein translocation appea...