Purification, refolding and characterization of the trimeric Omp2a outer membrane porin from Brucella melitensis

Brucella melitensis is a gram-negative bacteria known to cause brucellosis and to produce severe infections in humans. Whilst brucella’s outer membrane proteins have been extensively studied due to their potential role as antigens or virulence factors, their function is still poorly understood at the structural level, as the 3D structure of Brucella β-barrel membrane proteins are still unknown. In this context, the B. melitensis trimeric Omp2a porin has been overexpressed and refolded in n-dodecyl-β-d-maltopyranoside. We here show that this refolding process is insensitive to urea but is temperature- and ionic strength-dependent. Reassembled species were characterized by fluorescence, size-exclusion chromatography and circular dichroism. A refolding mechanism is proposed, suggesting that Omp2a first refolds under a monomeric form and then self-associates into a trimeric state. This first complete in vitro refolding of a membrane protein from B. melitensis shall eventually lead to functional and 3D structure determination.

► We overexpress a membrane protein in inclusion bodies in Escherichia coli.
► We refold Omp2a in vitro, an outer membrane protein from Brucella melitensis.
► n-Dodecyl-β-d-maltopyranoside is able to refold Omp2a.
► We obtain a pure folded sample in 4 days by removing urea and working at 40 °C, 0.5 mg/ml in 400 mM NaCl.
► Refolded Omp2a was biophysically charaterized as a trimeric β-barrel species.