PHALLOIDIN BINDING AND RHEOLOGICAL DIFFERENCES AMONG ACTIN ISOFORMS

Actin is a highly conserved protein in eukaryotes, yet different isofo rms of this protein can be found within the same cell. To begin to exp lore whether isoactin sequence diversity leads to functional differenc es in actin filaments, we have examined the phalloidin binding kinetic s and the bulk theologic properties of purified actin isoforms from a variety of eukaryotic sources. We observe differences in the phalloidi n association kinetics between muscle alpha- and cytoplasmic actins, P halloidin dissociates from all mammalian actin isoforms tested at the same slow rate, while dissociation from yeast actin is 1 order of magn itude more rapid. The actin isoforms form viscoelastic gels to varying degrees with skeletal muscle alpha-actin gels being the most elastic, smooch muscle alpha- and gamma-actins being less elastic, and beta-ac tin not forming elastic structures under our experimental conditions. The sequence variation among isoforms is discussed in light of these b iophysical and biochemical differences.