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Diverse effects of the MalE-LacZ hybrid protein on Escherichia coli cell physiology

The hybrid protein between the periplasmic maltose-binding protein and the cytoplasmic beta-galactosidase (the MalE-LacZ hybrid protein) was previously shown to block the export of envelope proteins when synthesized in large amounts. Now we show that the hybrid protein exerts another major effect on the cell, that is, induction of the heat shock proteins. This latter effect was dependent on the htpR gene product but independent of the function of the signal sequence on the hybrid protein. On the other hand, the previously reported induction of the SecA protein by the hybrid protein was independent of htpR and may be caused by the reduced protein export ability of the cell. The functional htpR gene is essential for viability of the cell in which the basal level of the hybrid protein is synthesized, whereas in the absence of the hybrid protein htpR is dispensable at low temperature. These results indicate that the hybrid protein somehow generates a signal or stress that is similar to what the cell experiences at elevated temperatures.