VirB2 and VirB5 proteins: specialized adhesins in bacterial type-IV secretion systems?

Many type-IV secretion systems (T4SSs) of plant and human pathogens assemble a pilus used to inject virulence molecules (effectors) into host target cells. The T4SS of Agrobacterium tumefaciens consists of VirB1-VirB11 and VirD4 proteins. Whether targeting of T4SSs to the host requires a T4SS-adhesin that specifically engages host receptors for delivery of effectors has, until recently, remained unclear. Recent data of Agrobacterium and Helicobacter indicate that two classes of T4SS components, VirB2 and VirB5, might function as adhesins that mediate host-cell targeting through binding to specific host receptors. Here, we discuss this important issue and recent progress in the field.