Translocators YopB and YopD from Yersinia form a multimeric integral membrane complex in eukaryotic cell membranes

The type-III secretion systems are contact-activated secretion systems that allow bacteria to inject effector proteins across eukaryotic cell membranes. The secretion apparatus, called injectisome or needle complex includes a needle that terminates with a tip structure. The injectisome exports its own distal components, like the needle subunit and the needle tip. Upon contact, it exports two hydrophobic proteins called translocators (YopB and YopD in Yersinia) and the effectors. The translocators, assisted by the needle tip, form a pore in the target cell membrane, but the structure of this pore remains elusive. Here we purified the membranes from infected sheep erythrocytes and we show that they contain integrated and not simply adherent YopB and YopD. In Blue Native PAGE, these proteins appeared as a multimeric 500-700 kDa complex. This hetero-polymeric YopBD complex could be co-purified after solubilization in 0.5 % dodecylmaltoside but not visualized in the electron microscope. We speculate that this complex may not be stable and rigid but only transient.