Modulation of actin polymerization by an exogenous protein, lysozyme.

Several methods (fluorescence, high- and low-shear viscosity, and electron microscopy) have been applied to measure the effects of lysozyme on actin polymerization. Under our conditions, at pH 8.0 and 20 degrees C, lysozyme is predominantly dimeric and its major effect is to inhibit the steady-state polymerization of actin. Those actin filaments formed in the presence of lysozyme are significantly shortened with recurrent amorphous densities along the filament length. However, at pH 6.4 and 37 degrees C, lysozyme is monomeric and actin filament cross-linking is observed. We reasoned that in hen egg white lysozyme the tripeptide L-arginyl-glycyl-aspartate (RGD), a sequence capable of mimicking a portion of the receptor sites of extracellular matrix proteins, might be important in lysozyme self-association and, therefore, actin-lysozyme interaction. The presence of RGD in the lysozyme-actin polymerizing 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 polymerization. Therefore, RGD most likely binds to a complementary RGD sequence on lysozyme and alters its ability to interact with actin and modify polymerization.