N-ETHYLMALEIMIDE-MODIFIED ACTIN-FILAMENTS DO NOT BUNDLE IN THE PRESENCE OF ALPHA-ACTININ

We show that the modification of actin subdomain 1 by N-ethylmaleimide (NEM), which binds Cys-374 close to the C-terminus of the molecule, i nhibits the a-actinin-induced bundling of actin filaments. This effect is not merely related to the block of Cys-374, since N-(1-pyrenyl)iod oacetamide (pyrene-IA) is unable to prevent bundling. Considering that NEM (but not pyrene-IA) influences actin assembly, we suggest that th e inhibition of the actin - alpha-actinin interaction is due to the ch emical modification of actin Cys-374 which, by inducing a marked spati al reorganization of actin monomers, is able to modify both the intra- and inter-molecular interactions of this protein. Finally, NEM-modifi ed actin filaments form bundles in the presence of polyethylene glycol 6000 since, in this case, the side by side association of actin filam ents does not depend on the accessibility of binding sites nor on the formation of chemical bonds.