Cellular response to accumulation of recombinant proteins in the E. coli inner membrane: Implications for proteolysis and productivity of the secretory expression system

Several literature reports have indicated adverse effects on host cell growth and physiology due to secretory expression of recombinant proteins. In this work, extensive proteolysis of recombinant streptokinase was observed in the secretory expression system, along with severe growth impairment and reduced productivity as compared to intracellular expression in Escherichia coli. These phenomena correlated well with the accumulation of the secretory recombinant protein in the inner membrane and a corresponding up-regulation of an inner-membrane protease (probably ClpX). This protease was found to be activated only in the post-induction phase of the secretory expression system. A reduction in the cultivation temperature led to a significant decrease in the accumulation level of recombinant streptokinase in the inner membrane and a concomitant decrease in the inner-membrane protease activity. This resulted in a considerable enhancement in the overall productivity of the secretory expression system. The results demonstrate that the productivity of the secretory expression system is controlled by the accumulation level of the recombinant protein in the inner membrane. Beyond a particular accumulation level the cellular response triggered in the form impaired growth and enhanced proteolysis significantly affects the productivity of the system. In order to reduce the accumulation in the inner membrane, the synthesis rate needs to be tuned to the protein-specific translocation efficiency of the cell.