Kinetic analysis of lamB mutants suggests the signal sequence plays multiple roles in protein export.
We have developed a quantitative assay to measure the rate of processing of precursor LamB into mature protein and have used this assay to characterize 10 previously isolated and 3 new lamB signal sequence mutants. The data suggest that the LamB signal sequence serves a complex function. Our assay has revealed five types of signal sequence defect: 1) a strong kinetic defect resulting from alteration of the secondary structure in the putative alpha-helical region in the hydrophobic core, 2) a strong, or 3) a weak kinetic defect due to placement of a charged residue in the hydrophobic core, 4) decreased synthesis of LamB, and 5) both a decrease in synthesis and a strong kinetic defect. The effect of an extragenic suppressor, prlA4 on the rate of processing pLamB containing signal sequence mutations was also examined and compared to the rates in wild-type strains. It was found that prlA4 increases the rate of processing in some, but not all, mutants having a kinetic defect while having no effect on the decreased synthesis seen in mutants of types 4 and 5.
D000483:Alleles; D000595:Amino Acid Sequence; D001425:Bacterial Outer Membrane Proteins / Q000235:genetics*; D001483:Base Sequence; D004926:Escherichia coli / Q000235:genetics*; D009154:Mutation; D018272:Porins; D021382:Protein Sorting Signals / Q000235:genetics*; D011991:Receptors, Virus / Q000235:genetics*
D001425:Bacterial Outer Membrane Proteins; D018272:Porins; D021382:Protein Sorting Signals; D011991:Receptors, Virus; C016532:maltoporins