The ability of calcium to regulate thin filament sliding velocity was studied in an in vitro motility assay system using cardiac troponin and tropomyosin and rhodamine-phalloidin-labeled skeletal actin and skeletal heavy meromyosin to propel the filaments. Measurements showed that both the number of thin filaments sliding and their sliding speed (Sf) were dependent on the calcium concentration in the range of pCa 5 to 9. Thin filament motility was completely inhibited only if troponin and tropomyosin were added at a concentration of 100 nM to the motility assay solution and the pCa was more than 8. The filament sliding speed was dependent on the pCa in a noncooperative fashion (Hill coefficient = 1) and reached maximum at 5 microns/s at a pCa of 5. The number of filaments moving uniformly decreased from > 90% at pCa 5-6 to near zero in less than 1 pCa unit. This behavior may be explained by a hypothesis in which the regulatory proteins control the number of cross-bridge heads interacting with the thin filaments rather than the rate at which they individually hydrolyze ATP or translocate the thin filaments.

Familial hypertrophic cardiomyopathy (HCM) can be caused by dominant missense mutations in cardiac troponin T (TnT), ␣ -tropomyosin, C-protein, or cardiac myosin heavy chain genes. The myosin mutations are known to impair function, but any functional consequences of the TnT mutations are unknown. This report describes the in vitro function of troponin containing an Ile91Asn mutation in rat cardiac TnT, corresponding to the HCM-causing Ile79Asn mutation in man. Mutant and wild-type TnT cDNAs were expressed in bacteria and the proteins purified and reconstituted with the other troponin subunits. The mutation had no effect on troponin's affinity for tropomyosin, troponininduced binding of tropomyosin to actin, cooperative binding of myosin subfragment 1 to the thin filament, Ca 2 ϩ -sensitive regulation of thin filament-myosin subfragment 1 ATPase activity, or the Ca 2 ϩ concentration dependence of this regulation. However, the mutation resulted in 50% faster thin filament movement over a surface coated with heavy meromyosin in in vitro motility assays. The increased sliding speed suggests an unexpected role for the amino terminal region of TnT in which this mutation occurs. The relationship between this faster motility and altered cardiac contraction in patients with HCM is discussed. ( J. Clin. Invest. 1996. 97:2842-2848.)

The function of three of tropomyosin's sequential quasiequivalent regions was studied by deletion from skeletal muscle ␣-tropomyosin of internal residues 49 -167. This deletion mutant tropomyosin spans four instead of the normal seven actins, and most of the tropomyosin region believed to interact with troponin is retained and uninterrupted in the mutant. The mutant tropomyosin was compared with a full-length control molecule that was modified to functionally resemble muscle tropomyosin (

Abbreviations used in this paper: +TIP, plus end -tracking protein; HSP, high speed pellet; SPB, spindle pole body; ts, temperature sensitive.

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