Role of membrane potential in protein folding and domain formation during secretion in escherichia coli

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Abstract
The synthesis and processing of the periplasmic components of the leucine transport system of E coli have been studied to determine the role played by transmembrane potential in protein secretion. Both the leucine‐isoleucine‐valine binding protein and the leucine‐specific binding protein are synthesized as precursors with 23 amino acid N‐terminal leader sequences. The processing of these precursors is sensitive to the transmembrane potential. Since the amino acid sequence and the crystal structure have been determined for the leucine‐isoleucine‐valine binding protein, it and the closely related leucine‐specific binding protein represent convenient models in which to examine the mechanism of protein secretion in E coli. A model for secretion has been proposed, suggesting a role for transmembrane potential. In this model, the N‐terminal amino acid sequence of the precursor is assumed to form a hairpin of two helices. The membrane potential may orient this structure to make it accessible to processing. In addition, the model suggests that a negatively charged, folded domain of the secretory protein may electrophorese toward the trans‐positive side of the membrane, thus providing an additional role for the transmembrane potential.