Chromatographic analysis of tropomyosins from rabbit skeletal, chicken gizzard and earthworm muscle.

Tropomyosins from rabbit skeletal, chicken gizzard and earthworm muscle all exist as dimeric, ca. 100% alpha-helical coiled-coil species in benign media. Two major tropomyosin isoforms from each muscle source have been identified and can be conveniently designated alpha (fast) and beta (slow) based on electrophoretic mobility under denaturing conditions. The ratio of alpha to beta chains is ca. 3-4:1 for rabbit skeletal and ca. 1:1 for chicken gizzard and earthworm tropomyosins. Each chain from the former two muscle sources has been sequenced, thus providing a molecular basis for interpreting the in vivo population of homo- and hetero-dimers. The characteristics of each purified tropomyosin in weak-anion exchange, strong-cation exchange and reversed-phase high-performance liquid chromatography are described. Binding to and/or elution from the reversed-phase matrix results in dissociation into highly helical monomeric chains. This mode of chromatography separates the alpha and beta chains of earthworm and chicken gizzard tropomyosins, but not those of the rabbit protein. Both anion- and cation-exchange chromatography use mild (benign) elution conditions under which the native, in vivo dimer population should be preserved. Only the rabbit protein exhibited peak separation on the anion-exchange resin, with peak assignment corresponding to the known molecular organization of homo- and hetero-dimers. In strong cation-exchange analysis, all three tropomyosins exhibit a chromatographic transition near pH 6.5, possibly the result of histidine(s) titration. Collectively, the chromatographic data confirm the present understanding of the in vivo mixture of dimers for tropomyosin from rabbit skeletal and chicken gizzard. It is concluded that native earthworm tropomyosin exists predominantly as an alpha beta hetero-dimer.