ACTIN FILAMENT ORGANIZATION IN ACTIVATED MAST-CELLS IS REGULATED BY HETEROTRIMERIC AND SMALL GTP-BINDING PROTEINS

Rat peritoneal mast cells, both intact and permeabilized, have been us ed widely as model secretory cells. GTP-binding proteins and calcium p lay a major role in controlling their secretory response. Here we have examined changes in the organization of actin filaments in intact mas t cells after activation by compound 48/80, and in permeabilized cells after direct activation of GTP-binding proteins by GTP-gamma-S. In bo th cases, a centripetal redistribution of cellular F-actin was observe d: the content of F-actin was reduced in the cortical region and incre ased in the cell interior. The overall F-actin content was increased. Using permeabilized cells, we show that AlF4-, an activator of heterot rimeric G proteins, induces the disassembly of F-actin at the cortex, while the appearance of actin filaments in the interior of the cell is dependent on two small GTPases, rho and rac. Rho was found to be resp onsible for de novo actin polymerization, presumably from a membrane-b ound monomeric pool, while rac was required for an entrapment of the r eleased cortical filaments. Thus, a heterotrimeric G-protein and the s mall GTPases, rho and rac, participate in affecting the changes in the actin cytoskeleton observed after activation of mast cells.