The Escherichia coli AIDA-I autotransporter undergoes cytoplasmic glycosylation independently of export.
The Escherichia coli adhesin involved in diffuse adherence (AIDA-I) is an outer membrane autotransporter protein and one of the few glycosylated proteins found in Escherichia coli. O-glycosylation is mediated by the product of the aah gene, which codes for a heptosyltransferase that uses ADP-glycero-manno-heptose precursors from the LPS biosynthesis pathway. Little else is known about Aah and mechanisms involved in modification of AIDA-I. We observed that Aah is mainly found in an insoluble fraction and, by deletion of the AIDA-I signal sequence or by blocking sec-translocation machinery with sodium azide, we showed that glycosylation occurs in the cytoplasm of bacteria independently of secretion. Since AIDA-I harbors an N-terminal extension in its signal sequence, we wondered whether glycosylation requires this unusual sequence. We observed that, while deletion of the N-terminal extension affected the expression level of AIDA-I, the protein was still exported to the outer membrane and glycosylated. Modification of a secreted protein in the cytoplasm raises several mechanistic questions.
Charbonneau
Marie-Eve
ME
Canada Research Chair on Bacterial Animal Diseases, Université de Montréal, Faculté de Médecine Vétérinaire, 3200 Sicotte, St-Hyacinthe, J2S 7C6, Québec, Canada.
Mourez
Michael
M
D018830:Adhesins, Escherichia coli / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D000595:Amino Acid Sequence; D003593:Cytoplasm / Q000235:genetics / Q000378:metabolism*; D004926:Escherichia coli / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D029968:Escherichia coli Proteins / Q000235:genetics / Q000378:metabolism; D006031:Glycosylation; D016695:Glycosyltransferases / Q000235:genetics / Q000378:metabolism; D008969:Molecular Sequence Data; D021382:Protein Sorting Signals; D021381:Protein Transport; D017384:Sequence Deletion
C075559:AIDA-I protein, E coli; D018830:Adhesins, Escherichia coli; D029968:Escherichia coli Proteins; D021382:Protein Sorting Signals; D016695:Glycosyltransferases; C433803:autotransporter-adhesin-heptosyltransferase, E coli