Brucella outer membrane protein Omp31 is a haemin-binding protein

DELPINO, M. V., CASSATARO, J., FOSSATI, C. A., GOLDBAUM, F. A., AND BALDI, P. C.

MICROBES AND INFECTION

Año: 2006 vol. 8 p. 1203 - 1208

1286-4579

The expression of haemin-binding proteins (HBPs) in the outer membrane is one of the strategies used by Gram-negative bacteria to obtainiron from the host. No HBP has been described in Brucella spp. We investigated whether Omp31, an outer membrane protein from Brucella withhomology to HBPs from Bartonella quintana, is an HBP. Soluble recombinant Omp31 bound specifically to haemin-agarose, while an unrelatedBrucella protein (SurA) did not. A similar experiment showed that native Omp31 found in the Brucella suis membrane fraction also binds tohaemin-agarose. Recombinant Omp31 was electrophoresed by SDS-PAGE, transferred to nitrocellulose, and incubated with a haemin solution.Haemin bound to Omp31 and to albumin (positive control) but not to SurA. IPTG-induced recombinant Escherichia coli cells expressing Omp31on their membrane bound significantly more haemin than uninduced cells or controls carrying a similar plasmid without the omp31 gene, showingthat Omp31 also binds haemin in a bacterial membrane environment. Viable Brucella ovis cells bound haemin in solution, and this bindingwas markedly inhibited by preincubation of cells with antibodies to Omp31 and to an exposed prominent loop of the protein, thus showing thatOmp31 functions as an HBP in brucellae. To test whether the expression of Omp31 is iron-regulated, B. suis was grown in trypticase-soy broth(TSB) and in iron-depleted TSB. The expression of Omp31, as assessed by Western blot, was significantly higher in bacteria grown under ironlimitation. Overall, these results show that Omp31 from B. suis, B. melitensis and B. ovis is an HBP, whose expression seems to be induced byiron limitation.