Functional implications of secondary structure analysis of wild type and mutant bacterial signal peptides

The N-terminal extension found in the precursor form of many exported proteins in bacteria--the so called signal peptide--plays an essential role in the exportation process. Analysis of the secondary structure of several bacterial signal peptides by predictive methods shows that the predicted structures are essentially periodical and define a preferential axis. We assume that at least one step in export depends directly on the length of the hydrophobic core of the signal peptide measured along this axis. We call this parameter the Hydrophobic Axis Length or HAL. Examination of the available mutations located in bacterial signal peptides and blocking export permits us to define a minimum functional HAL value (the threshold HAL) under which little or no export occurs. A mutation of the lipoprotein signal peptide leaves an HaL above the threshold: this accounts readily for the fact that export of the protein is not strongly affected. The value calculated for the threshold HAL, ie, 18 A, corresponds to half the size of the lipid bilayer of the membrane.