Pertactin beta-helix folding mechanism suggests common themes for the secretion and folding of autotransporter proteins.
Many virulence factors secreted from pathogenic Gram-negative bacteria are autotransporter proteins. The final step of autotransporter secretion is C --> N-terminal threading of the passenger domain through the outer membrane (OM), mediated by a cotranslated C-terminal porin domain. The native structure is formed only after this final secretion step, which requires neither ATP nor a proton gradient. Sequence analysis reveals that, despite size, sequence, and functional diversity among autotransporter passenger domains, >97% are predicted to form parallel beta-helices, indicating this structural topology may be important for secretion. We report the folding behavior of pertactin, an autotransporter passenger domain from Bordetella pertussis. The pertactin beta-helix folds reversibly in isolation, but folding is much slower than expected based on size and native-state topology. Surprisingly, pertactin is not prone to aggregation during folding, even though folding is extremely slow. Interestingly, equilibrium denaturation results in the formation of a partially folded structure, a stable core comprising the C-terminal half of the protein. Examination of the pertactin crystal structure does not reveal any obvious reason for the enhanced stability of the C terminus. In vivo, slow folding would prevent premature folding of the passenger domain in the periplasm, before OM secretion. Moreover, the extra stability of the C-terminal rungs of the beta-helix might serve as a template for the formation of native protein during OM secretion; hence, vectorial folding of the beta-helix could contribute to the energy-independent translocation mechanism. Coupled with the sequence analysis, the results presented here suggest a general mechanism for autotransporter secretion.
Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, IN 46556-5670, USA.
Mirco
M
Junker
Christopher C
CC
Schuster
Andrew V
AV
McDonnell
Kelli A
KA
Sorg
Mary C
MC
Finn
Bonnie
B
Berger
Patricia L
PL
Clark
D001425:Bacterial Outer Membrane Proteins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D002462:Cell Membrane / Q000378:metabolism; D026901:Membrane Transport Proteins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D008958:Models, Molecular; D008970:Molecular Weight; D011485:Protein Binding; D011489:Protein Denaturation / Q000187:drug effects; D017510:Protein Folding; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary; D019032:Spectrometry, Mass, Matrix-Assisted Laser Desorption-Ionization; D014357:Trypsin / Q000737:chemistry / Q000378:metabolism; D010566:Virulence Factors, Bordetella / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism
D001425:Bacterial Outer Membrane Proteins; D026901:Membrane Transport Proteins; D010566:Virulence Factors, Bordetella; C068929:pertactin; D014357:Trypsin