Signal sequences often containa-helix-destabilizing amino acids within the hydrophobic core. In the precursor of the Escherichia coliouter-membrane protein PhoE,the glycine residue at position)10 (Gly )10 ) is thought to be responsible for the break in the a-helix. Previously,we showed that substitution of Gly )10 bya-helix-promoting residues (Ala, Cys or Leu) reduced the proton-motive force dependency of the translocation of the precursor,but the actual role of the helix breaker remained obscure. .
