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Pertactin Beta-Helix Folding Mechanism Suggests Common
Themes for the Secretion and Folding of Autotransporter Proteins
Mirco Junker, Christopher Schuster, Andrew McDonnell,
Kelli Sorg, Mary Finn, Bonnie Berger & Patricia Clark
Abstract
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 co-translated 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 amongst 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,
prior to 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.
References:
[1] Mirco Junker, Christopher C. Schuster, Andrew V. McDonnell, Kelli
A. Sorg, Mary C. Finn, Bonnie Berger and Patricia L. Clark. Pertactin
Beta-Helix Folding Mechanism Suggests Common Themes for the Secretion
and Folding of Autotransporter Proteins. Proceedings of the National Academy
of Sciences (2006), 103: 4918-4923.
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