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A conformational landscape for alginate secretion across the outer membrane of Pseudomonas aeruginosa

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Description:

The exopolysaccharide alginate is an important component
of biofilms produced by
Pseudomonas aeruginosa
, a major
pathogen that contributes to the demise of cystic fibrosis
patients. Alginate exits the cell
via
the outer membrane porin
AlgE. X-ray structures of several AlgE crystal forms are
reported here. Whilst all share a common
-barrel constitu-
tion, they differ in the degree to which loops L2 and T8 are
ordered. L2 and T8 have been identified as an extracellular
gate (E-gate) and a periplasmic gate (P-gate), respectively,
that reside on either side of an alginate-selectivity pore
located midway through AlgE. Passage of alginate across the
membrane is proposed to be regulated by the sequential
opening and closing of the two gates. In one crystal form,
the selectivity pore contains a bound citrate. Because citrate
mimics the uronate monomers of alginate, its location is taken
to highlight a route through AlgE taken by alginate as it
crosses the pore. Docking and molecular-dynamics simula-
tions support and extend the proposed transport mechanism.
Specifically, the P-gate and E-gate are flexible and move
between open and closed states. Citrate can leave the
selectivity pore bidirectionally. Alginate docks stably in a
linear conformation through the open pore. To translate
across the pore, a force is required that presumably is
provided by the alginate-synthesis machinery. Accessing the
open pore is facilitated by complex formation between AlgE
and the periplasmic protein AlgK. Alginate can thread
through a continuous pore in the complex, suggesting that
AlgK pre-orients newly synthesized exopolysaccharide for
delivery to AlgE.

Suggested citation:

. () A conformational landscape for alginate secretion across the outer membrane of Pseudomonas aeruginosa [Online]. Available from: http://publichealthwell.ie/node/877721 [Accessed: 19th September 2019].

  

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