Differential regulation of skeletal muscle myosin‐II and brush border myosin‐I enzymology and mechanochemistry by bacterially produced tropomyosin isoforms

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Abstract
In this report, we have compared the physical properties and actin‐binding characteristics of several bacterially produced nonmuscle and striated muscle tropomyosins, and we have examined the effects of these isoforms on the interactions of actin with two structurally distinct classes of myosin: striated muscle myosin‐II and brush border (BB) myosin‐I. All of the bacterially produced nonmuscle tropomyosins bind to F‐actin with the expected stoichiometry and with affinities comparable to that of a tissue produced α‐tropomyosin, although the striated muscle tropomyosin CTm7 has a lower affinity of F‐actin than a tissue‐purified striated muscle α tropomyosin. The bacterially produced isoforms also protect F‐actin from severing by villin as effectively as tissue‐purified striated muscle α‐tropomyosin. The bacterially produced 284 amino acid striated muscle tropomyosin isoform CTm7, the 284 amino acid nonmuscle tropomyosin isoform CTm4, and two chimeric tropomyosins (CTm47 and CTm74) all inhibit the actin‐activated MgATPase activity of muscle myosin S1 by ∼ 70–85%, comparable to the inhibition seen with tissue‐purified striated muscle α tropomyosin. The 248 amino acid tropomyosin XTm4 stimulated the actin‐activated MgATPase activity of muscle myosin S1 approximately two‐ to threefold. The in vitro sliding of actin filaments translocated by muscle myosin‐II (2.4 μm/sec at 19°C, 5.0 μm/s at 24°C) increased 25–65% in the presence of XTm4. Tropomyosins CTm4, CTm7, CTm47, and CTm74 had no detectable effect on myosin‐II motility. The actin‐activated MgATPase activity of BB myosin‐I was inhibited 75–90% by all of the tropomyosin isoforms tested, including the 248 amino acid tropomyosin XTm4. BB myosin‐I motility (50 nm/s) was completely inhibited by both the 248 and 284 amino acid tropomyosins. These results demonstrate that bacterially produced tropomyosins can differentially regulate myosin enzymology and mechanochemistry, and suggest a role for tropomyosin in the coordinated regulation of myosin isoforms in vivo.