Ser(756) of beta 2 integrin controls Rap1 activity during inside-out activation of alpha M beta 2

During alpha M beta 2-mediated phagocytosis, the small GTPase Rap1 activates the beta 2 integrin by binding to a region between residues 732 and 761. Using COS-7 cells transfected with alpha M beta 2, we show that alpha M beta 2 activation by the phorbol ester PMA involves Ser(756) of beta 2. This residue is critical for the local positioning of talin and biochemically interacts with Rap1. Using the CaM (calmodulin) antagonist W7, we found Rap1 recruitment and the inside-out activation of alpha M beta 2-mediated to be affected. We also report a role for CaMKII (calcium/CaM-dependent kinase II) in the activation of Rap1 during integrin activation. These results demonstrate a distinct physiological role for Ser(756) of beta 2 integrin, in conjunction with the actions of talin and Rap1, during alpha M beta 2-mediated activation in macrophages.