MODULATION OF ACTIN POLYMERIZATION BY AN EXOGENOUS PROTEIN, LYSOZYME

Several methods (fluorescence, high- and low-shear viscosity, and elec tron microscopy) have been applied to measure the effects of lysozyme on actin polymerization. Under our conditions, at pH 8.0 and 20-degree s-C, lysozyme is predominantly dimeric and its major effect is to inhi bit the steady-state polymerization of actin. Those actin filaments fo rmed in the presence of lysozyme are significantly shortened with recu rrent amorphous densities along the filament length. However, at pH 6. 4 and 37-degrees-C, lysozyme is monomeric and actin filament cross-lin king is observed. We reasoned that in hen egg white lysozyme the tripe ptide L-arginyl-glycyl-aspartate (RGD), a sequence capable of mimickin g a portion of the receptor sites of extracellular matrix proteins, mi ght be important in lysozyme self-association and, therefore, actin-ly sozyme interaction. The presence of RGD in the lysozyme-actin polymeri zing solutions at pH 8.0 and 20-degrees-C caused an inhibition of the dimeric lysozyme effects, while RGD alone had no effects on actin poly merization. Therefore, RGD most likely binds to a complementary RGD se quence on lysozyme and alters its ability to interact with actin and m odify polymerization.