Interaction of phalloidin with chemically modified actin

Modification of Tyr‐69 with tetranitromethane impairs the polymerizability of actin in accordance with the previous report [Lehrer, S. S. and Elzinga, M. (1972) Fed. Proc. 31 , 502]. Phalloidin induces this chemically modified actin to form the same characteristic helical thread‐like structure as normal F‐actin. The filaments bind myosin heads and activate the myosin ATPase activity as effectively as normal F‐actin. When a dansyl group is introduced at the same point [Chantler, P. D. and Gratzer, W. B. (1975) Eur. J. Biochem. 60 , 67–72], phalloidin still induces the polymerization. The filaments bind myosin heads and activate the myosin ATPase activity. These results indicate that Tyr‐69 is not directly involved in either an actin‐actin binding site or the myosin binding site on actin. Moreover, the results suggest that phalloidin binds to actin monomer in the presence of salt and its binding induces a conformational change in actin which is essential for polymerization, or that actin monomer fluctuates between in unpolymerizable and polymerizable form while phalloidin binds to actin only in the polymerizable form and its binding locks the conformation which causes the irreversible polymerization of actin. Modification of Tyr‐53 with 5‐diazonium‐(1 H )tetrazole blocks actin polymerization [Bender, N., Fasold, H., Kenmoku, A., Middelhoff, G. and Volk, K. E. (1976) Eur. J. Biochem. 64 , 215–218]. Phalloidin is unable to induce the polymerization of this modified actin nor does it bind to it. Phalloidin does not induce the polymerization of the trypsin‐digested actin core. These results indicate that the site at which phalloidin binds is involved in polymerization and the probable conformational change involved in polymerization may be modulated through this site.