Rap1: Selective activator of β2 integrins?

In T cells, Rap1 is well-recognized as a major mediator of inside-out signaling to integrins following stimulation by antigen and chemokines. Ghandour and colleagues have identified a selective role for Rap1 in human T cells in chemokine-dependent adhesion via LFA-1, but not VLA-4. Their study highlights the specificity of Rap1 for LFA-1 in SDF-1α-mediated adhesion, and shows that PLC and the Rap1 exchanger CalDAG-GEFI are required.