In common with selected species of Listeria, Shigella, Rickettsia and Mycobacterium, some members of the genus Burkholderia are capable of intracellular actin-based motility. In the case of B. pseudomallei this requires BimA, which exhibits C-terminal homology to the Yersinia autosecreted adhesin YadA and shares motifs with cellular actin-binding proteins and nucleation-promoting factors. Our research has established that tandem WASP-homology (WH2) domains of B. pseudomallei BimA (BimABp) are required to bind and polymerise actin monomers in a manner independent of the cellular Arp2/3 complex. Direct repeats of a casein kinase II target site, which vary in number in natural isolates of B. pseudomallei, were found to be required for actin-based motility and act additively to promote actin polymerization.
Functional BimA homologues exist in the glanders pathogen B. mallei (BimABm) and avirulent saprophyte B. thailandensis (BimABt). BimABt appears distinct from other BimA variants in that it possesses a conserved central acidic (CA) domain. Such domains often mediate the recruitment and activation of the Arp2/3 complex. Consistent with this prediction, BimABt was found to require Arp2/3 for actin-polymerization and in-frame deletion of the CA domain of BimABt abolished this activity and the binding of the Arp2/3 complex components. These data suggest that different mechanisms for actin-based motility have evolved even among closely-related Burkholderia species.
Our preliminary data indicate that BimA may be modified post-translationally by phosphorylation and glycosylation. Indeed, a predicted glycosyltransferase encoded immediately 5’ of bimA in B. pseudomallei (BPSS1491) is required for actin-based motility but not expression or polar localization of BimA and ongoing work aims to define the nature and consequences of glycan modification. We have also identified cellular proteins that are recruited to the surface of BimA-expressing bacteria by mass spectrometry and are dissecting the protein-protein interactions required for actin-based motility of B. pseudomallei.