its affinity for actin. Fluorescently labeled phalloid ins stain F-actin at nanomolarconcentrations. Labeled phalloidins
have similar affinity for both large and small filaments, binding in a stoichiometric ratio of about one phalloid in
molecule per actin subunit in muscle and non-muscle cells from various species of plants and animals.
Different from antibodies, the binding affinity of phalloid in does not change significantly with actin among different
species.Non-specific staining is negligible, and the contrast between stained and unstained areas is extremely large.
Phalloid in shifts the monomer/polymer equilibrium toward the polymer, lowering the critical concentration for
polymerization up to 30-fold. Phallotoxins also stabilize F-actin, inhibiting depolymerization by cytochalasin, potassium iodide and elevated temperatures. Because the phalloidin conjugates are small, with an approximate diameter of 12-15?
and molecular weight of <2000 Daltons, a variety of actin-binding proteins including myosin, tropomyosin and troponin
can still bind to actin after treatment with phalloidin. Even more significantly, phalloid in-labeled actin filaments remain
functional; labeled glycerinated muscle fibers still contract, and labeled actin filaments still move on solid-phase myosin
substrates. Fluorescent phalloid in can also be used to quantify the amount of F-actin in cells.
