A signal sequence is not required for protein export in prlA mutants of Escherichia coli.
The prlA/secY gene, which codes for an integral membrane protein component of the Escherichia coli protein export machinery, is the locus of the strongest suppressors of signal sequence mutations. We demonstrate that two exported proteins of E.coli, maltose-binding protein and alkaline phosphatase, each lacking its entire signal sequence, are exported to the periplasm in several prlA mutants. The export efficiency can be substantial; in a strain carrying the prlA4 allele, 30% of signal-sequenceless alkaline phosphatase is exported to the periplasm. Other components of the E.coli export machinery, including SecA, are required for this export. SecB is required for the export of signal-sequenceless alkaline phosphatase even though the normal export of alkaline phosphatase does not require this chaperonin. Our findings indicate that signal sequences confer speed and efficiency upon the export process, but that they are not always essential for export. Entry into the export pathway may involve components that so overlap in function that the absence of a signal sequence can be compensated for, or there may exist one or more means of entry that do not require signal sequences at all.
Derman
A I
AI
Department of Microbiology and Molecular Genetics, Harvard Medical School, Boston, MA 02115.
Puziss
J W
JW
Bassford
P J
PJ
Beckwith
J
J
D018528:ATP-Binding Cassette Transporters; D000469:Alkaline Phosphatase / Q000378:metabolism*; D001426:Bacterial Proteins / Q000235:genetics*; D001483:Base Sequence; D001692:Biological Transport / Q000235:genetics; D002352:Carrier Proteins / Q000378:metabolism*; D004269:DNA, Bacterial; D004926:Escherichia coli / Q000201:enzymology / Q000235:genetics / Q000378:metabolism*; D029968:Escherichia coli Proteins; D058619:Maltose-Binding Proteins; D008969:Molecular Sequence Data; D009004:Monosaccharide Transport Proteins; D009154:Mutation; D011499:Protein Processing, Post-Translational; D021382:Protein Sorting Signals / Q000235:genetics / Q000378:metabolism*; D000069816:SEC Translocation Channels
D018528:ATP-Binding Cassette Transporters; D001426:Bacterial Proteins; D002352:Carrier Proteins; D004269:DNA, Bacterial; D029968:Escherichia coli Proteins; D058619:Maltose-Binding Proteins; D009004:Monosaccharide Transport Proteins; D021382:Protein Sorting Signals; D000069816:SEC Translocation Channels; C057841:SecY protein, E coli; C461599:maltose transport system, E coli; D000469:Alkaline Phosphatase