LATERAL MOBILITY OF INTEGRIN ALPHA(IIB)BETA(3) (GLYCOPROTEIN IIB IIIA) IN THE PLASMA-MEMBRANE OF A HUMAN MEGAKARYOCYTE/

The migration of integrins to sites of cell-cell and cell-matrix conta ct is thought to be important for adhesion strengthening. We studied t he lateral diffusion of integrin alpha(IIb)beta(3) (glycoprotein IIb/I IIa) in the plasma membrane of a cultured human megakaryocyte by fluor escence recovery after photobleaching of FITC-labelled monovalent Fab fragments directed against the beta(3) subunit. The diffusion of beta( 3) on the unstimulated megakaryocyte showed a lateral diffusion coeffi cient (D) of 0.37 x 10(-9) cm(2)/s and a mobile fraction of about 50%. Stimulation with ADP (20 mu M) or alpha-thrombin (10 U/ml) at 22 degr ees C induced transient decreases in both parameters reducing D to 0.2 1 x 10(-9) cm(2)/s and the mobile fraction to about 25%. The fall in D was observed within 1 min after stimulation but the fall in mobile fr action showed a lag phase of 5 min. The lag phase was absent in the pr esence of Calpain I inhibitor, whereas cytochalasin D completely aboli shed the decrease in mobile fraction. The data are compatible with the concept that cell activation induces anchorage of 50% of the mobile a lpha(IIb)beta(3) (25% of the whole population of receptor) to the cyto plasmic actin filaments, although, as discussed, other rationals are n ot ruled out.