Rap1 controls activation of the α(M)β(2) integrin in a talin-dependent manner.

The small GTPase Rap1 and the cytoskeletal protein talin regulate binding of C3bi-opsonised red blood cells (RBC) to integrin α(M)β(2) in phagocytic cells, although the mechanism has not been investigated. Using COS-7 cells transfected with α(M)β(2), we show that Rap1 acts on the β(2) and not the α(M) chain, and that residues 732-761 of the β(2) subunit are essential for Rap1-induced RBC binding. Activation of α(M)β(2) by Rap1 was dependent on W747 and F754 in the β(2) tails, which are required for talin head binding, suggesting a link between Rap1 and talin in this process. Using talin1 knock-out cells or siRNA-mediated talin1 knockdown in the THP-1 monocytic cell line, we show that Rap1 acts upstream of talin but surprisingly, RIAM knockdown had little effect on integrin-mediated RBC binding or cell spreading. Interestingly, Rap1 and talin influence each other's localisation at phagocytic cups, and co-immunoprecipitation experiments suggest that they interact together. These results show that Rap1-mediated activation of α(M)β(2) in macrophages shares both common and distinct features from Rap1 activation of α(IIb)β(3) expressed in CHO cells.