dnak and dnaj heat shock proteins participate in protein export in escherichia coli in escherichia coli secreted proteins must be maintained in an exportcompetent state before translocation across the cytoplasmic membrane this function is carried out by a group of proteins called chaperones secb is the major chaperone that interacts with precursor proteins before their secretion we report results indicating that the dnak and dnaj heat shock proteins are also involved in the export of several proteins most likely by acting as their chaperones translocation of alkaline phosphatase a secbindependent protein was inhibited in dnak and dnaj mutant strains suggesting that export of this protein probably involves dnak and dnaj in addition dnak and dnaj play a critical role in strains lacking secb they are required both for viability and for the residual processing of the secbdependent proteins lamb and maltosebinding protein mbp seen in secb null strains furthermore overproduction of dnak and dnaj permits strains lacking secb to grow in rich medium and accelerates the processing of lamb and mbp these results suggest that under conditions where secb becomes limiting dnak and dnaj probably substitute for secb and facilitate protein export this provides the cell with a mechanism to overcome a temporary imbalance in the secretion process caused by an abrupt expansion in the pool of precursor proteins