FIGURE 3.

The translocation pathways for the surface exposure of lipoproteins in Brucella. Prelipoproteins synthesized in the cytoplasm undergo lipid modification generating a mature, triacylated lipoprotein. Brucella lipoproteins (orange, green, and blue structures) may be released from the inner membrane (IM) to the outer membrane (OM) using the lipoprotein localization machinery (Lol) pathway, which is propelled by Adenosine triphosphate (ATP hydrolysis), an ATP binding cassette (ABC) transporter (LolCDE) and a carrier protein (LolA). LolB is absent in α-Proteobacteria, as such it is proposed lipoprotein flippase located in the OM transfers lipoproteins to the OM. Another proposed translocation pathway involves the export of the Brucella lipoproteins through the β- barrel domain of β- barrel assembly complex (Bam) complex embedded in the OM, assisted by the periplasmic chaperones (SurA) to the surface of the OM. The Brucella Bam complex lacks the BamB and BamC components found in other gram negative bacteria, however, a BamF component with a conserved sequence motif related to the BamC component may perform a similar role. Abbreviations used: inner membrane (IM), outer membrane (OM), lipoprotein localization machinery (Lol), adenosine triphosphate (ATP), ATP binding cassette (ABC), β- barrel assembly complex (Bam).