Importance of secondary structure in the signal sequence for protein secretion.

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Abstract
Mutant Escherichia coli strains in which export of the LamB protein (coded for by the lamB gene) to the outer membrane of the cell is prevented were described previously. One of these mutant strains contains a small (12-base pair) deletion mutation within the region of the lamB gene that codes for the NH2-terminal signal sequence. In this mutant strain, export but not synthesis of the LamB protein is blocked. One isolated pseudorevertants that restore export of functional LamB protein to the outer membrane. DNA sequence analysis showed that 2 of the revertants contain a point mutation in addition to the original deletion. These point mutations lead to amino acid substitutions within the signal sequence. These secondary mutations efficiently suppress the export defect caused by the deletion mutation. Analysis of the secondary structure of the wild-type, mutant and pseudorevertant LamB signal sequences suggests that the secondary mutations restore export by allowing the formation of a stable .alpha.-helical conformation in the central, hydrophobic region of the signal sequence.