Maturation of Escherichia coli maltose-binding protein by signal peptidase I in vivo. Sequence requirements for efficient processing and demonstration of an alternate cleavage site.
Comparative analyses of a number of secretory proteins processed by eukaryotic and prokaryotic signal peptidases have identified a strongly conserved feature regarding the residues positioned -3 and -1 relative to the cleavage site. These 2 residues of the signal peptide are thought to constitute a recognition site for the processing enzyme and are usually amino acids with small, neutral side chains. It was shown previously that the substitution of aspartic acid for alanine at -3 of the Escherichia coli maltose-binding protein (MBP) signal peptide blocked maturation by signal peptidase I but had no noticeable effect or MBP translocation across the cytoplasmic membrane of its biological activity. This identified an excellent system in which to undertake a detailed investigation of the structural requirements and limitations for the cleavage site. In vitro mutagenesis was used to generate 14 different amino acid substitutions at -3 and 13 different amino acid substitutions at -1 of the MBP signal peptide. The maturation of the mutant precursor species expressed in vivo was examined. Overall, the results obtained agreed fairly well with statistically derived models of signal peptidase I specificity, except that cysteine was found to permit efficient processing when present at either -3 and -1, and threonine at -1 resulted in inefficient processing. Interestingly, it was found that substitutions at -1 which blocked processing at the normal cleavage site redirected processing, with varying efficiencies, to an alternate site in the signal peptide represented by the Ala-X-Ala sequence at positions -5 to -3. The substitution of aspartic acid for alanine at -5 blocked processing at this alternate site but not the normal site. The amino acids occupying the -5 and -3 positions in many other prokaryotic signal peptides also have the potential for constituting alternate processing sites. This appears to represent another example of redundant information contained within the signal peptide.
Fikes
J D
JD
Department of Microbiology and Immunology, School of Medicine, University of North Carolina, Chapel Hill 27599-7290.
Barkocy-Gallagher
G A
GA
Klapper
D G
DG
Bassford
P J
PJ
D018528:ATP-Binding Cassette Transporters; D000595:Amino Acid Sequence; D001483:Base Sequence; D002352:Carrier Proteins / Q000235:genetics* / Q000302:isolation & purification; D010450:Endopeptidases / Q000378:metabolism*; D004926:Escherichia coli / Q000235:genetics* / Q000378:metabolism; D029968:Escherichia coli Proteins; D008320:Maltose / Q000378:metabolism*; D058619:Maltose-Binding Proteins; D008565:Membrane Proteins / Q000235:genetics; D008969:Molecular Sequence Data; D009004:Monosaccharide Transport Proteins; D009154:Mutation; D015345:Oligonucleotide Probes; D011498:Protein Precursors / Q000235:genetics / Q000378:metabolism; D011499:Protein Processing, Post-Translational; D012697:Serine Endopeptidases
D018528:ATP-Binding Cassette Transporters; D002352:Carrier Proteins; D029968:Escherichia coli Proteins; D058619:Maltose-Binding Proteins; D008565:Membrane Proteins; D009004:Monosaccharide Transport Proteins; D015345:Oligonucleotide Probes; D011498:Protein Precursors; C461599:maltose transport system, E coli; D008320:Maltose; D010450:Endopeptidases; D012697:Serine Endopeptidases; C026515:type I signal peptidase