Rat mucosal-type mast cells of the RBL-2H3 line express a glycoprotein termed the MAst cell Function-associated Antigen (MAFA). When MAFA is clustered by its specific monoclonal antibody G63, secretion normally triggered by aggregating these cells' type I Fcepsilon receptor (FcepsilonRI) is substantially inhibited. The nature of MAFA-FcepsilonRI interactions giving rise to this inhibition remains unclear. Rotational diffusion of a membrane protein is a sensitive probe of its involvement in intermolecular interactions. We have therefore studied by time-resolved phosphorescence anisotropy the rotational behavior of both MAFA and FcepsilonRI as ligated by various reagents involved in FcepsilonRI-induced degranulation and MAFA-mediated inhibition thereof. From 4 to 37 degrees C, the rotational correlation times (mean +/- SD) of FcepsilonRI-bound, erythrosin-conjugated IgE resemble those observed for MAFA-bound, erythrosin-conjugated G63 Fab, 82 +/- 17 and 79 +/- 31 micros at 4 degrees C, respectively. Clustering the FcepsilonRI-IgE complex by antigen or by anti-IgE increases the phosphorescence anisotropy of G63 Fab and slows its rotational relaxation. Lateral diffusion of G63 Fab is also slowed by antigen clustering of the receptor. Taken together, these results indicate that unperturbed MAFA associates with clustered FcepsilonRI. They are also consistent with its interaction with the isolated receptor.