Small-molecule transport by CarO, an abundant eight-stranded β-barrel outer membrane protein from Acinetobacter baumannii

Michael Zahn, Tommaso D'agostino, Elif Eren, Arnaud Baslé, Matteo Ceccarelli, Bert Van Den Berg

Research output: Contribution to journalArticlepeer-review


Outer membrane (OM) β-barrel proteins composed of 12–18 β-strands mediate cellular entry of small molecules in Gram-negative bacteria. Small OM proteins with barrels of 10 strands or less are not known to transport small molecules. CarO (carbapenem-associated outer membrane protein) from Acinetobacter baumannii is a small OM protein that has been implicated in the uptake of ornithine and carbapenem antibiotics. Here we report crystal structures of three isoforms of CarO. The structures are very similar and show a monomeric eight-stranded barrel lacking an open channel. CarO has a substantial extracellular domain resembling a glove that contains all the divergent residues between the different isoforms. Liposome swelling experiments demonstrate that full-length CarO and a “loop-less” truncation mutant mediate small-molecule uptake at low levels but that they are unlikely to mediate passage of carbapenem antibiotics. These results are confirmed by biased molecular dynamics simulations that allowed us to quantitatively model the transport of selected small molecules.
Original languageEnglish
Pages (from-to)2329-2339
JournalJournal of Molecular Biology
Issue number14
Early online date3 Apr 2015
Publication statusPublished - 1 Jul 2015


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